Class 12 Physics Chapter 9 Important Questions Ray Optics and Optical Instruments

Q 1:- A light ray falls on a mirror normally. What are the values of the angle of incidence and the angle of reflection. Ans:- For a light ray falling

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Ray Optics and Optical Instruments. The best solution of the problem is to practice as many Physics Class 12 Chapter 9 Important Questions as possible to clear the doubts.

Q 1:- A light ray falls on a mirror normally. What are the values of the angle of incidence and the angle of reflection.
Ans:- For a light ray falling normally on a mirror, the values of both angles of incidence and reflection are zero.

Q 2:- What is the focal length and power of a plane mirror?
Ans:- The focal length of a plane mirror is infinite and its power is zero.

Q 3:- Out of speed, frequency, wavelength and colour of light, which parameter(s) remain same even after reflection from a curved mirror?
Ans:- All the four parameters namely speed, frequency, wavelength and colour of light remain unchanged when light beam undergoes reflection.

Q 4:- When does a concave mirror form a virtual image?
Ans:- A concave mirror forms a virtual, erect and magnified image of an object when it is placed between pole and principal focus of the concave mirror.

Q 5:- What is the difference between virtual images of an object formed by a concave mirror and a convex mirror?
Ans:- The virtual image of an object formed by a concave mirror is always magnified one but the image formed by a convex mirror is always diminished one.

Q 6:- Can a virtual image be obtained on a screen? Can a virtual image be photographed ?
Ans:- A virtual image cannot be obtained on a screen. However, a virtual image can be photographed.

Q 7:- Which light ray, for a spherical mirror, retraces its path after reflection? Why?
Ans:- A light ray passing through the centre of curvature (of a concave mirror) or towards the centre of curvature (of a convex mirror) exactly retraces its path after reflection because angle of incidence \angle i is zero for this ray.

Q 8:- Which mirror acts as a convergent mirror? Which mirror acts as a divergent mirror?
Ans:- A concave mirror acts as a convergent mirror but a convex mirror acts as a divergent mirror.

Q 9:- What does positive/negative sign of linear magnification signify?
Ans:- The positive sign of linear magnification signifies that the image of an object is erect one. On the other hand, negative sign of linear magnification means that the image is an inverted image.

Q 10:- Define refractive index of a medium. What is its SI unit?
Ans:- The refractive index of a given medium is defined as the ratio of the speed of light in free space (vaccum) to the speed of light in the given medium. Refractive index is a pure ratio having no units and dimensions.

Q 11:- Define refractive index of one medium with respect to another medium in terms of wavelength.
Ans:- The refractive index of a medium B w.r.t. another transparent medium A is defined as the ratio of wavelength of a given light beam in medium A to the wavelength of that very light beam in second medium. Mathematically,

n_{BA}=\frac{\lambda_A}{\lambda_B}

Q 12:- For two given media, how are the refractive indices n_{21} and n_{12} related to?
Ans:- Either we can say that

n_{21}=\frac{1}{n_{12}}

Or we can say that n21 × n12 = 1

Q 13:- For which medium is refractive index (a) minimum and (b) maximum
Ans:- (a) The refractive index for free space is minimum having a value 1.

(b) The refractive index for diamond is maximum having a value 2.42 .

Q 14:- A light ray falls normally on air - water interface. What are the angle of incidence and angle of refraction? What is the angle of deviation?
Ans:- For normal incidence on air- water interface, the angle of incidence, the angle of refraction and angle of deviation all are zero.

Q 15:- For what angle of incidence, the lateral shift produced by a rectangular glass slab is zero?
Ans:- When angle of incidence is zero.

Q 16:- What is critical angle ? Does its value, for a given pair of media, remain constant for light of all colours?
Ans:- The critical angle, for a given pair of media, is the angle of incidence in optically denser medium for which the angle of refraction in the rarer medium is 90° (or the refracted ray travels grazing along the interface). The value of critical angle, for a given pair of media, depends on the wavelength or colour of light. In fact, value of critical angle is the least for violet light and maximum for red light.

Q 17:- What is total internal reflection of light?
Ans:- Total internal reflection is the phenomenon of complete reflection of light taking place when a light ray travelling in an optically denser medium is incident at its interface with a rarer medium at an angle greater than the critical angle for that pair of media.

Q 18:- Can total internal reflection take place when light travels from rarer to a denser medium?
Ans:- No, it is not possible.

Q 19:- Name two examples of total internal reflection.
Ans:- Brilliance of diamond, mirage and action of optical fibre are common examples of total internal reflection of light.

Q 20:- Name the physical principle on which working of an optical fibre is based.
Ans:- Working of an optical fibre is based on the principle of total internal reflection.

Q 21:- What is a lens? What is its function?
Ans:- A lens is a part of a refracting medium bounded by two nonparallel curved surfaces or one curved and another plane surface. A lens either converges or diverges a light beam passing through it.

Q 22:- Can a lens be used in a material medium of which it is made of?
Ans:- No, a lens cannot be used in a material medium of which it is made of, because then there is no change in medium and hence no converging or diverging of incident light beam.

Q 23:- What is the focal length of a plane glass plate?
Ans:- The focal length of a plane glass plate is infinite.

Q 24:- What is the minimum distance between an object and its real image formed by a convex lens?
Ans:- The minimum distance between an object and its real image formed by a convex lens must be four times of its focal length.

Q 25:- Power of a convex lens is taken +ve but that of concave lens is taken -ve. Why?
Ans:- Focal length of a convex lens is having +ve sign. Consequently, its power P=\frac{1}{f}, too has a positive sign. A concave lens has negative focal length and therefore, its power is also negative.

Q 26:- A convex lens is combined with a concave lens of same focal length. What is the power and focal length of the combination?
Ans:- When a convex lens of focal length +f is combined with a concave lens of focal length -f, the power of the combination is zero and focal length of the combination is infinite.

Q 27:- Which mirror has a positive power and which mirror has a negative power?
Ans:- A concave mirror has a positive power and a convex mirror has a negative power.

Q 28:- Out of red and violet lights, for which colour is the refractive index of glass greater?
Ans:- The refractive index of glass is maximum for violet colour and minimum for red colour. In fact, refractive index of glass gradually decreases with increase in wavelength of light.

Q 29:- On what factors does the angle of deviation suffered by a light ray passing through a prism depend?
Ans:- The angle of deviation of a light ray on emergence from a prism depends on the material of prism, angle of prism and the angle of incidence of light ray at the prism.

Q 30:- How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light?
Ans:- The angle of minimum deviation of a glass prism decreases if the incident violet light is replaced by red light.

Q 31:- Write the relation for the refractive index of the prism in terms of the angle of minimum deviation and the angle of prism.
Ans:- Refractive index of prism, n=\frac{sin(\frac{A+\delta_m}{2})}{sin(\frac{A}{2})} Where A = angle of prism and delta_m= angle of minimum deviation

Q 32:- A glass lens of refractive index 1.45 disappears when immersed in a liquid. What is the value of refractive index of the liquid
Ans:- The value of refractive index of the liquid is 1.45, that is, exactly the same as that of glass.

Q 33:- Which medium can cause dispersion of light? Can air cause dispersion?
Ans:- Only a dispersive medium can cause dispersion of light. A dispersive medium is that in which lights of different wavelengths travel with different speeds and hence suffer different values of web deviations. Air is not a dispersive medium and thus cannot cause dispersion of light.

Q 34:- On what factors does angular dispersion produced by a prism depend?
Ans:- The angular dispersion produced by a prism depends on the material of the prism and the angle of prism.

Q 35:- Under what condition does the formation of rainbow occur?
Ans:- The conditions for formation or observing a rainbow are that the Sun should be shining in one part of the sky (say, near western horizon) while it is raining in the opposite part of the sky (say, eastern horizon). Therefore, an observer can see a rainbow only when his back is towards the Sun.

Q 36:- What are the values of near point and far point for a normal eye?
Ans:- For a normal eye, the near point is 25 cm away from the eye and the far point is at infinity.

Q 37:- The diameter of the planet Jupiter is much more than that of even our Earth. Even then, in night sky, its size appears to be extremely small. Why?
Ans:- Although size (diameter) of the planet Jupiter is large, yet its distance from the Earth is much much more. As a result, visual angle subtended by Jupiter at the eye is extremely small and consequently, it appears to be extremely small.

Q 38:- What is visual angle?
Ans:- Visual angle is the angle subtended by an object at the eye.

Q 39:- What are two common causes of myopia? How can it be corrected?
Ans:- Two common causes of myopia defect are

(a) excessive curvature (less focal length) of eye lens, and

(b) elongation of eyeball. Myopia can be corrected by using concave lens. If far point of defective eye is x, then one should use concave lens of focal length f = -x cm for correction of the defect.

Q 40:- What are two common causes of hypermetropia? How can it be corrected?
Ans:- Two common causes of hypermetropia defect are

(a) less curvature (higher focal length) of eye lens, and

(b) shortening of size of eyeball. Hypermetropia can be corrected by using convex lens of Al appropriate focal length or power.

Q 41:- A person looking at a grid of lines can focus sharply in the horizontal plane but cannot focus so sharp in the vertical plane. With what defect of vision is he/she suffering and what is the remedy?
Ans:- The person concerned is suffering with astigmatism. He should use cylindrical lens of suitable radius of curvature with an appropriately directed axis.

Q 42:- What is short-sightedness (myopia)? What is the type of lens used to correct it?
Ans:- Short-sightedness (myopia) is a defect of vision in which the affected person can see nearby objects clearly but cannot see distinctly the objects situated far away from the eye. For rectification of short-sightedness one should use concave (diverging) lens of an appropriate power.

Q 43:- What is long-sightedness (hypermetropia)? What is the type of lens used to correct it?
Ans:- Long-sightedness (hypermetropia) is a defect of vision in which the affected person can see far-off objects clearly but cannot see distinctly the objects situated near the eye. For rectification of long-sightedness, one should use convex (converging) lens of an appropriate power.

Q 44:- On which principle does a simple magnifier work?
Ans:- If an object is placed between optical centre and principal focus of a convex lens, its virtual, erect and magnified image is formed on the same side of the lens.

Q 45:- For a simple microscope, should one prefer a lens of higher/smaller focal length, and why?
Ans:- For a simple microscope, one prefers a lens of smaller focal length so as to have higher value of magnifying power.

Q 46:- What is normal adjustment of a microscope or telescope? What is its main advantage?
Ans:- By normal adjustment of a microscope or telescope, we mean that the final image is being formed at infinity. When a person works in normal adjustment of microscope or telescope, the eye is relaxed and there is no strain on the eye.

Q 47:- What are the characteristics of the final image formed by a compound microscope at the near point of eye?
Ans:- The image formed is virtual, highly magnified and inverted as compared to the original object.

Q 48:- What is the distance between the objective and eyepiece lenses of an astronomical telescope in normal adjustment?
Ans:- In normal adjustment of an astronomical telescope, the distance between the objective and eyepiece lenses is (fo+fe), i.e., equal to sum of the focal lengths of the two lenses.

Q 49:- Why is the aperture of objective lens of an astronomical telescope so large?
Ans:- The aperture of objective lens of an astronomical telescope is kept large so as to increase its resolving power as well as its light gathering power.

Q 50:- Can we increase the range of a telescope by increasing the diameter of the objective lens? How?
Ans:- Yes, we can. On increasing the diameter of the objective lens of a telescope, its light gathering power is increased. Thus, the telescope enables us to see even those stars which are situated farther off. Effectively, it means that the range of telescope has increased.

Q 51:- How does the magnifying power of a telescope change on increasing or decreasing the aperture of its objective lens?
Ans:- On increasing or decreasing the aperture of objective lens of a telescope, its magnifying power remains unchanged. The magnifying power of a telescope simply depends on the focal lengths of objective and eyepiece lenses.

Q 52:- Write two merits of a reflecting type telescope over a refracting type telescope.
Ans:- (a) The objective reflector of a reflecting type telescope is free from chromatic aberration. A paraboloid-shaped reflector is also free from spherical aberration. Thus, quality of image formed by a reflecting type telescope is much better than that for a refracting type telescope.

(b) It is easier to prepare a large-sized reflector mirror than an objective lens of same size. Moreover, mechanical support system of reflecting telescope is less cumbersome and easier to operate .

Q 53:- Which is more readily seen at a distance, a white blotting paper or a polished mirror? Why?
Ans:- A white blotting paper is more readily visible from a distance than a polished mirror because the surface of white blotting paper provides irregular reflection and hence light spreads in all possible directions and one can see the paper from a distance. A polished mirror gives rise to only regular reflection and can be seen only in the direction of reflected rays. Hence, it cannot be seen from a distance easily.

Q 54:- Does size of a reflecting mirror affect the nature and size of the image?
Ans:- No, size of a reflecting mirror does not affect either the nature or the size of the image of an object placed in front of it. Of course, intensity or brightness of image depends on the size of mirror and state of its polish.

Q 55:- Why are mirrors used in searchlights (or car headlights) parabolic in shape and not concave spherical?
Ans:- A spherical mirror of large aperture suffers with the spherical aberration. However, a parabolic reflector is free from spherical aberration. Therefore, when a source of light is placed at the focus point of a parabolic mirror, it produces a perfect parallel beam of ligh which is visible even from a long distance.

Q 56:- Although a virtual image cannot be produced on a screen, we can see it. How?
Ans:- The virtual image acts as an object for the eye lens of the observer. The eye lens, in turn, produces a real image on retina. Thus, one can see even the virtual image of object.

Q 57:- Suppose that the lower half of the concave mirror's reflecting surface is covered with an opaque (non reflective) material. What effect will this have on the image of an object placed in front of the mirror?
Ans:- Applying the laws of reflection to remaining upper half of the concave mirror's reflecting surface, we find that the mirror will even now form the image of the whole object. However, as the area of the reflecting surface has been reduced to one-half, the intensity of the image will be low. (In fact, half of the intensity of that obtained with full mirror).

Q 58:- How does the frequency of a beam of ultraviolet light change when it goes from air into glass?
Ans:- The frequency of beam of ultraviolet light remains unchanged when it goes from air into glass. Due to change in medium, speed and wavelength of light change but frequency does not change at all.

Q 59:- A light ray, while undergoing refraction at the interface of two transparent media, bends towards the normal in the second medium. What does it signify?
Ans:- The bending of light towards the normal on entering into the second medium signifies that the second medium is optically denser having higher value of refractive index and speed of light in second medium is lesser than that in first medium.

Q 60:- When light travels from a rarer to denser medium, the speed decreases. Does the decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer.
Ans:- When light travels from a rarer to denser medium, the speed of light photon (E = hv, where v = frequency and h = planck's constant) remains unchanged. Consequently, energy carried by the light wave remains unchanged.

Q 61:- When monochromatic light travels from one medium to another, its wavelength changes but frequency remains the same. Explain.
Ans:- When monochromatic light travels from one medium to another, its speed and correspondingly, its wavelength changes but frequency of light remains unchanged. It is because frequency is the most inherent property of light wave, which depends only on the atomic or molecular structure of substance emitting the light.

Q 62:- Will a star twinkle if seen from space (say, moon or a spacecraft)?
Ans:- No, a star will not appear twinkling if seen from free space because there will be no atmospheric refraction. The star will appear to be shining continuously.

Q 63:- Why is lens maker's formula called so?
Ans:- For a lens of the given focal length and given material, by using lens maker's formula, we can calculate values of radii of curvature of two surfaces of lens. Then, by grinding the two surfaces of a piece of plane glass, its two surfaces can be given the desired radii of curvature and lens can be prepared. It is this reason due to which the formula is called the lens maker's formula.

Q 64:- How does the intensity of image formed by a lens depend on its aperture?
Ans:- In reference to lens system, aperture means the effective diameter of its light transmitting area. Hence, the intensity of image formed by a lens, which depends on the light passing through the lens, is directly proportional to the square of its aperture.

Q 65:- Violet colour is seen at the bottom of the spectrum, when white light is dispersed by prism. Why?
Ans:- The wavelength of violet-coloured light is the least out of various colours of white light and refractive index of the prism is maximum for violet light. As a result, the violet colour ray suffers maximum deviation and is seen at the bottom of a spectrum formed by a prism.

Q 66:- Does the dispersive power of a prism depend on the size, shape or angle of the prism ? Does it depend on angle of incidence or the angle of emergence of light ray?
Ans:- No, the dispersive power of a prism is a characteristics of its material and is independent of all other factors like shape or size of prism or angle of prism or angle of incidence or angle of emergence etc.

Q 67:- Can a single prism cause deviation of a white light ray without causing any dispersion? Can a combination of two prisms can do that? How?
Ans:- A single prism cannot cause deviation for a white ray of light without simultaneously causing dispersion of light. However, a combination of two prisms can do that. We select two prisms that dispersion caused by first prism is nullified by second prism but deviations produced by them are not nullified. So that, there is some finite deviation. Such a prism combination is called an achromatic prism combination.

Q 68:- A ray of white light shows no dispersion on emerging from a glass slab or plate. Explain why.
Ans:- When a ray of white light is incident obliquely on a glass slab or plate the white light ray is splitted up into its constituent colours. Hence, inside the glass or slab, dispersion is taking place. But at the second surface of a slab, the emergent rays corresponding to all the colours are exactly parallel to each other on account of the fact that two refracting faces of a slab or plate are parallel to each other and hence, the angle of emergence \angle e= angle of incidence \angle i. As a result, the emergent light shows no dispersion and appears white.

Q 69:- Why are haloes (rings) observed around the Moon sometimes?
Ans:- When the Moon is seen through a thin veil of high clouds, haloes are formed. The clouds contain a large number of icy crystals. The haloes are formed due to reflection of light from these icy crystals.

Q 70:- What is chromatic aberration? What is its cause?
Ans:- The image of an object in white light, formed by a lens, is usually coloured and blurred. This defect of image is called chromatic aberration. The aberration arises due to the fact that focal length of the lens is minimum for violet colour and maximum for red colour.

Q 71:- What for are the optical instruments generally used?
Ans:- Optical instruments are commonly used for the following purposes:

(a) To produce the image of a close lying tiny object at the least distance of distinct vision of eye so that it is seen distinctly.

(b) To increase the visual angle at the eye so that the object can be seen in more details.

(c) To improve upon the resolving power of observer's eye.

Q 72:- The nature has provided us two eyes. What is the importance of this gift of nature to us?
Ans:- The two eyes enable us to perceive, besides length and breadth, even the depth of the space. In this manner, we have a three dimensional view of the world around us. If we had been provided only one eye by nature, it would not have been possible to have the idea of the distance of an object from ourselves.

Q 73:- What is the difference between hypermetropia and presbyopia?
Ans:- Persons suffering with either hypermetropia or presbyopia can see distant object clearly but cannot see objects situated near the eye i.e., in both cases, the near point of eye is shifted away from its normal value of 25 cm. However, the cause of these two defects of vision is different. Hypermetropia is caused due to less curvature of cornea as a result of which focal length of the eye lens system increases or its converging power decreases. On the other hand, presbyopia is caused due to decreasing effectiveness of the ciliary muscles and loss of flexibility of the eye lens with growing age. Both these defects of vision can be corrected by using convex lenses of appropriate power.

Q 74:- If a telescope is inverted, will it serve as a compound microscope?
Ans:- No, an inverted telescope will not serve at all as a compound microscope.

Q 75:- How can you distinguish between a telescope and a compound microscope simply by seeing them?
Ans:- The length of the tube (i.e., distance between objective and eyepiece lenses) for a compound microscope is kept fixed but length of the tube of a telescope is adjustable. Thus, a device with fixed tube length is a compound microscope and device with adjustable tube length is a telescope.

Q 76:- The magnifying power of a simple microscope is inversely proportional to the focal length of the lens. What then stops us from using a convex lens of smaller and smaller focal length and achieving greater and greater magnifying power?
Ans:- Yes, magnifying power of a simple microscope is inversely proportional to its focal length. Yet we cannot increase the magnification beyond a further limit owing to the following reasons:

(a) It is extremely difficult to grind a convex lens of extremely short focal length.

(b) The lens of short focal length becomes a thick lens, which does not form a sharp image of an object due to various defects of images present in a thick lens.

Q 77:- In viewing through a magnifying glass, one usually positions one's eyes very close to the lens. Does angular magnification change if the eye is moved back ?
Ans:- The angular magnification slightly decreases if the eye is moved away from the magnifying glass. Reason for this is that the angle subtended by the image at the eye would be slightly less than the angle subtended by the image at the lens.

Q 78:- For which position of the object a convex lens forms an image of exactly the same size? What is its use?
Ans:- When an object is placed laterally on the principal axis of a convex lens at a distance 2f, then its real and inverted image of exactly the same size is formed on other side of the lens at a distance 2f from the lens. This property of a convex lens is used by an inverting lens which inverts the object without causing any magnification.

Q 79:- Under what condition is a prism not visible when dipped in a liquid? What will be the angle of deviation suffered by a light ray in that situation?
Ans:- The prism is not visible if it is immersed in a liquid whose refractive index is same as that of prism material. In that situation, a light ray will go straight through the prism and will not suffer any deviation.

Q 80:- We read a book because of the light that it reflects. Then, why do not we see even a faint image of ourselves in the book?
Ans:- Yes, of course, we are able to read a book on account of irregular reflection (or diffused reflection) of light from it. But irregular reflection does not produce an image. An image is produced only when regular reflection or refraction of light takes place.

Q 81 :- An air bubble in water shines brilliantly and appears silvery. Why?
Ans:- An air bubble in water shines brilliantly due to total internal reflection at its outer surface. Here, light is propagating from water (optical denser medium) to air (optically rarer medium). Therefore, if as shown in the following figure, the incident ray subtends an angle i > ic (Where ic, for water-air interface is 48.6°), the light is totally internally reflected. As a result, the surface of air bubble appears silvery one.

Q 82:- Why do we prefer the objective lens of a focal length even smaller than that of eyepiece in a compound microscope?
Ans:- In a compound microscope, although fo, and fe both are small, but fo is still smaller than fe. This is done to reduce the length of the microscope tube. We know that the object is placed in front of objective lens at a distance slightly greater than its focal length and the objective lens forms its real and inverted image on other side of the lens at a distance much beyond 2fo. If we use an objective lens of higher focal length, the image will be formed still farther away and the length of microscope tube would become much longer. The larger size of the microscope tube, beyond a limit, reduces the intensity of the final image formed by ibe the microscope.

Q 83:- In microscopes and telescopes, we generally use two or more lenses in contact to prepare objective or eyepiece lenses. Why?
Ans:- By having a combination of two or more lenses in contact, we can design lenses of desired values of focal lengths. We can also eliminate chromatic as well as spherical aberrations by this method. The arrangement helps in increasing the angular magnification of instrument as well as its field of view.

Q 84:- Give the ratio of velocities of light rays of wavelength 4000 A and 8000 A in vaccum.
Ans:- The ratio is one, because light rays of all the wavelength travel in vaccum with a constant speed of 3 × 108 m/s.

Q 85:- How is the power of a mirror related to its focal length?
Ans:- The power of a mirror is the negative of the reciprocal of its focal length expressed in metre. For a mirror of focal length f(m), the power is given by

P=-\frac{1}{f(m)}

Q 86:- Objective and eyepiece of a compound microscope are marked 15 x and 10 x respectively. What does it mean?
Ans:- Marking of 15 x and 10 x on objective and eyepiece means that magnifications of these two lenses are 15 and 10 respectively. Thus, the total magnification achieved by compound microscope is given by-

m = mo x me = 15 x 10 = 150

Q 87:- How can you distinguish between a plane mirror, a concave mirror and a convex mirror without actually touching them?
Ans:- We can identify a mirror by simply viewing our image formed by it. If one stands near a plane mirror, he sees an erect image of same size. As the person moves away from the mirror, the image remains erect and of same size.

If one stands near a concave mirror, the image formed is erect and enlarged. As the person moves away then beyond particular position, the image becomes an a inverted image. If one stands near a convex mirror, he sees an erect and diminished image. As the person moves away, the erect image becomes more and more diminished.

Q 88:- How is the focal length of a spherical mirror affected when the wavelength of the light used is increased?
Ans:- There is no change in the focal length of a spherical mirror when the wavelength of light used is increased. It is because reflection takes place from a mirror and reflection phenomenon does not involve a change in medium. Thus, refractive index of a medium or change in wavelength has no relevance.