RAY OPTICS

The speed of light in an isotropic medium depends on,

• its intensity

• its wavelength

• the nature of propagation

• the motion of the source w.r.t medium

A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is

• 2.5 cm

• 5 cm

• 10 cm

• 15 cm

Answer explanation

An object is placed in front of a convex mirror of focal length of f and the maximum and minimum distance of an object from the mirror such that the image formed is real and magnified.

• 2f and c

• c and ∞

• f and O

• None of these

Answer explanation

None of these 

There is no maximum minimum object distance for convex mirror to form real and inverted image.

For light incident from air on a slab of refractive index 2, the maximum possible angle of refraction is,

• 30⁰

• 45⁰

• 60⁰

• 90⁰

If the velocity and wavelength of light in air is V_a and λ_a and that in water is V_w and λ_w , then the refractive index of water is,

Stars twinkle due to

• reflection

• total internal reflection

• refraction

• polarisation

Answer explanation

When a ray of light travels from one medium to another it ‘bends’. This phenomenon is referred to as refraction. If it travels from a rare medium to a dense medium, it bends towards the normal and if it travels from a dense medium to a rarer medium, it bends away from the normal. The speed at which the light travels changes depending on the medium and therefore this bending occurs.

This effect can be observed when light passes through a prism or a glass slab and even when light passes through water. The light ray travels from air to a medium of different densities here.

So how are refraction and twinkling connected? The atmosphere of the earth is made of different layers. It is affected by winds, varying temperatures, and different densities as well. When light from a distant source (a star) passes through our turbulent (moving air) atmosphere, it undergoes refraction many times. When we finally perceive this light from a star, it appears to be twinkling! This is because some light rays reach us directly, and some bends away from and toward us. It happens so fast that it gives a twinkling effect.

When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. T his implies that the liquid must have refractive index,

• less than one

• less than that of glass

• greater than that of glass

• equal to that of glass

The radius of curvature of curved surface at a thin planoconvex lens is 10 cm and the refractive index is 1.5. If the plane surface is silvered, then the focal length will be

• 5 cm

• 15 cm

• 10 cm

• 20 cm

An air bubble in glass slab of refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness of the slab is,

• 8 cm

• 10 cm

• 12 cm

• 16 cm

Answer explanation

A ray of light travelling in a transparent medium of refractive index n falls, on a surface separating the medium from air at an angle of incidents of 45^o. The ray can undergo total internal reflection for the following n,

• n = 1.25

• n = 1.33

• n = 1.4

• n = 1.5

Answer explanation