Light 3

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41. Can you ever have situation in which light ray goes undeviated through a prism? Explain.

No, the situation is not found where the light ray goes undeviated through a prism. The main property of the prism is that the net deviation produced by the prism is not zero. As the angle incidence of the prism increases, the deviation first decreases, becomes minimum and then increases. Hence, light rays are always deviated through a prism when they pass through the prism.

42. What are grazing incidence and grazing emergence?

In a prism, the deviation is maximum when the angle of incidence is right angled and in this case, the ray has grazing incidence on the first refracting face of the prism. This is known as grazing incidence. According to the principle of the reversibility of light, it follows that if the ray of light is incident on the first face at angle i, the angle of emergent on the other face is right angle. This is grazing emergence.

43. A convex lens is immersed in water. Will its focal length change?

As the Lens Maker formula for a convex lens is given by:

Thus, the refractive index of the lens depends not only on the radii of curvature of lens but also on the refractive index of medium in which it is placed. As the lens is immersed in water, the value of refractive index decreases. Hence, the focal length of the convex lens will increase if it is immersed in water.

44. What are the advantages of total reflecting prism over plane mirror?

In case of reflection from plane mirror, all of light is not reflected due to the intensity of reflected light is equal to the intensity of incident light in case of total internal reflection but not in case of reflection from plane mirror. In addition, the reflection through a thick mirror causes multiple image formation as well as lateral inversion of image. By the criticism obtained from plane mirror, the total reflecting prism is used.

45. Can you think of a situation in which a convex (converging) lens would behave like a concave (diverging) lens?

Yes, there is a situation in which a convex ( converging ) lens would behave like a concave ( diverging ) lens. When a convex lens of glass (refractive index = 1.5) is immersed in the glycerin (refractive index = 1.67) then it will behave like a diverging lens.

46. The image of a candle formed by a convex lens is obtained on a screen. Will the full size of the image be obtained if the lower half the lens is painted black and completely opaque?

The image of a candle formed by a convex lens is obtained on a screen. Yes, the full size of the image will be obtained of lesser intensity than it ought to be if the lower half the lens is painted black and also if completely opaque.

47. A lens when immersed in a transparent liquid is not visible. Under what conditions can this happen?

A lens when immersed in a transparent liquid is not visible. This happens when the absolute refractive index of both lens and liquid is equal.

48. What will happen if a lens is immersed in a liquid whose absolute refractive index is equal to that of the lens?

When a lens is immersed in a liquid whose absolute refractive index is equal to that of the lens then the lens makers formula becomes:

or, f = infinite

So, focal length (f) = infinite

Hence, the lens will behave as a plane glass if it is immersed in a liquid whose refractive index is equal to that of the lens.

49. Under what condition does a concave lens form a real image?

If the object is virtual, a concave lens forms a real image. A virtual object means the rays of light from the object are converging. So, if the incident rays to concave lens are converging in nature, the lens may form a real image.

50. Sunglasses have curved surfaces. But their power is zero. Why?

For the sun glasses, the curved surfaces have their center of curvature on the same side and their radii are equal. So, the Lens maker formula becomes:

This is the reason why the sun glasses have zero power.

51. How will you combine a convex lens and a concave lens such that the combination acts as a plane glass plate?

If the focal length of convex lens is equal to the focal length of the concave lens, the combination of lenses behaves as a plane glass plate because for this type of combination the net power of combination is zero or null and the focal length of the plane glass is infinite by the lens formula:

52. An air bubble is formed inside water. Does it act as a converging lens or a diverging lens?

If an air bubble is formed inside water that is spherical in shape whose surfaces are convex to water (denser medium) and concave to air (rarer medium) then the surface convex to water always diverge light on it and acts as a diverging lens. Hence, an air bubble inside water behaves as a diverging lens.

53. Does the focal length of a lens depend on the material of the lens?

As the focal length of a lens depends upon the refractive index of the material which depends on the material by which lens is made by the Lens makers formula:

Hence, the focal length of a lens depends on the material of the lens.

54. Why does a lens refract?

When the light passes from one medium to another then the velocity of light is also changed. Due to this change in velocity of light, it bends and refraction is possible. For a light to pass from air to lens, there are two media in which the velocity of the light is different. Hence, it follows the laws of refraction and a lens refract light.

55. You are given a lens though you can see but you are not allowed to touch. How would you test whether it is convex or concave?

If the object is kept very close to the lens and viewed from the other side, if the image appears to be erect and magnified then the lens is convex and if the image is erect and diminished in size, the lens is concave.

56. What will happen if a lens is immersed in a liquid whose absolute refractive index is less than to that of the lens?

When a lens is immersed in a liquid whose absolute refractive index is less than that of the lens, then the focal length of it becomes positive which makes the lens to behave as a converging lens according to the Lens Maker formula:

57. What will happen if a lens is immersed in liquid whose absolute refractive index is greater to that of the lens?

When a lens is immersed in a liquid whose absolute refractive index is greater than that of the lens, then the focal length of it becomes negative which makes the lens to behave as a diverging lens according to the Lens Maker formula:

58. How can you obtain a real image by using a convex mirror? Is it magnified?

when a convergent beam of light falls on a convex mirror, the object for the mirror will be virtual and the image will be real. It is not magnified.

59. Explain why can a mirror not give rise to chromatic aberration?

While light passes from air to a lens, lights of different wavelength have different speeds. They are deviated by different angles which give rise to a chromatic aberration. While the light gets reflected from the surface of a mirror, it reflect back to air. So, lights of all wavelengths travel with the same speed and there is no chromatic aberration.

60. Which travels more slowly in glass: red light or violet light?

As the value of the refractive index µ for the red light is minimum due to its maximum value of the wavelength and the value of the refractive index µ for the violet light is maximum due to its minimum value of the wavelength. The velocity of the light in the glass medium is given by

Where, c is the velocity of light in air or vacuum & is the refractive index of the glass.

Hence from above relation, red light travels more faster than the violet light.

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