A thin plano-convex lens acts like a concave mirror of focal length 0.2 m when silvered from its plane surface. The refractive index of the material of the lens is 1.5. The radius of curvature of the convex surface of the lens will be

• 0.1 m

• 0.75 m

• 0.4 m

• 0.2 m

92.

The refractive index of a particular material is 1.67 for blue light, 1.65 for yellow light and 1.63 for red light. The dispersive power of the material is

• 0.031

• 1.60

• 0.0615

• 0.024

Rainbow is formed due to

• total internal reflection

• scattering

• refraction

• dispersion and total internal reflection

A beam of parallel rays is brought to focus by a plano-convex lens. A thin concave lens of the same focal length is joined to the first lens. The effect of this is

• the focus shifts to infinity

• the focal point shifts towards the lens by a small distance

• the focal point shifts away from the lens by a small distance

• the focus remains undisturbed

Which mirror is to be used to obtain a parallel beam of light from a small lamp ?

• Plane mirror

• Convex mirror

• Concave mirror

• Any one of these

Which of the following is a wrong statement ?

• D = 1/f where f is the focal length and D is called the refractive power of a lens

• Power is expressed in a diopter when f is in metres

• Power is expressed in diopter and does not depend on the system of unit used to measure f

• D is positive for convergent lens and negative for divergent lens

Identify the wrong description of the above figures :

• 1 represents far-sightedness

• 2 correction for short sightedness

• 3 represents far-sightedness

• 4 correction for far-sightedness

The wave theory of light, in its original form, was first postulated by

• Isaac Newton

• Chnstian Huygens

• Thomas Young

• Augustin Jean Fresnel

At Kavalur in India, the astronomers using a telescope whose objective had a diameter of 1 m started using telescope of diameter 2.54 m. This resulted in

• the increase in the resolvmg power by 2.54 times for the same λ.

• the increase in the limiting angle by 2.54 times for the same λ.

• decrease in the resolving power

• no effect on the limiting angle

The twinkling effect of star light is due to

• total internal reflection

• high dense matter of star

• constant burning of hydrogen in the star

• the fluctuating apparent position of the star being slightly different from the actual position of the star