How does it behaves when a convex lens is kept in a medium whose refractive index is greater than that of the medium?

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?

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?

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 &

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

Option 3 : a divergent lens

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60 Questions 60 Marks 60 Mins

Concept:

• Concave lens: It is a diverging lens that diverging the parallel beam of light.
  • It can also gather light from all directions and project it as a parallel beam.
  • The focal length of the concave lens is negative.
  • It has a virtual focus from the diverging rays of light that seem to converge.
• Convex lens: The lens whose refracting surface is upside is called a convex lens.
  • The convex lens is also called a converging lens.
  • The focal length of a convex lens is positive.

Explanation:

• The µ of the convex lens is greater than air so it will act as a convergent lens in the air.
• When it is dipped in water its µ is less than water. Now the rays entering and exiting the lens will bend toward or away from the normal in an opposite fashion than when the lens was placed in air.
• So the convergent lens in the air will now behave as a divergent lens in the water.

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