Science (English Medium) Class 12 - CBSE Important Questions

Case study: Mirage in deserts

To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage. This type of mirage is especially common in hot deserts. Based on the above facts, answer the following question:

The following figure shows a cross-section of a ‘light pipe’ made of a glass fiber of refractive index 1.68. The outer covering of the pipe is made of a material of refractive index 1.44. What is the range of the angles of the incident rays with the axis of the pipe for the following phenomena to occur.

Read the following paragraph and answer the questions.

1. Why the image formed at infinity is often considered most suitable for viewing. Explain
2. In modern microscopes, multicomponent lenses are used for both the objective and the eyepiece. Why?
3. Write two points of difference between a compound microscope and an astronomical telescope
   OR
   Write two distinct advantages of a reflecting type telescope over a refracting type telescope.

With the help of a ray diagram, show how a compound microscope forms a magnified image of a tiny object, at least distance of distinct vision. Hence derive an expression for the magnification produced by it.

Define the critical angle for a given pair of media and total internal reflection. Obtain the relation between the critical angle and refractive index of the medium.

A point object in the air is placed symmetrically at a distance of 60 cm in front of a concave spherical surface with a refractive index of 1.5. If the radius of curvature of the surface is 20 cm, find the position of the image formed.

• Assertion (A): Propagation of light through an optical fibre is due to total internal reflection taking place at the core-cladding interface.
• Reason (R): Refractive index of the material of the cladding of the optical fibre is greater than that of the core.

Show that the least possible distance between an object and its real image in a convex lens is 4f, where f is the focal length of the lens.

In an astronomical telescope in normal adjustment, a straight black line of length L is drawn on the objective lens. The eyepiece forms a real image of this line whose length is 𝑙. What is the angular magnification of the telescope?

A converging lens has a focal length of 10 cm in air. It is made of a material with a refractive index of 1.6. If it is immersed in a liquid of refractive index 1.3, find its new focal length.

Draw the sketches to differentiate between plane wavefront and spherical wavefront.

Three identical polaroid sheets P₁, P₂ and P₃ are oriented so that the pass axis of P₂ and P₃ are inclined at angles of 60° and 90° respectively with the pass axis of P₁. A monochromatic source S of unpolarised light of intensity I₀ is kept in front of the polaroid sheet P₁ as shown in the figure. Determine the intensities of light as observed by the observer at O, when polaroid P₃ is rotated with respect to P₂ at angles θ = 30° and 60°.

The intensity at the central maxima in Young’s double slit experiment is I₀. Find out the intensity at a point where the path difference is` lambda/6,lambda/4 and lambda/3.`

Write the important characteristic features by which the interference can be distinguished from the observed diffraction pattern.

Show, using a simple polaroid, that light waves are transverse in nature. Intensity of light coming out of a polaroid does not change irrespective of the orientation of the pass axis of the polaroid. Explain why.

In young’s double slit experiment, deduce the conditions for obtaining constructive and destructive interference fringes. Hence, deduce the expression for the fringe width.

Show that the fringe pattern on the screen is actually a superposition of slit diffraction from each slit.

What should be the width of each slit to obtain 10 maxima of the double slit pattern within the central maximum of the single slit pattern, for green light of wavelength 500 nm, if the separation between two slits is 1 mm?

Two monochromatic rays of light are incident normally on the face AB of an isosceles right-angled prism ABC. The refractive indices of the glass prism for the two rays '1' and '2' are respectively 1.3 and 1.5. Trace the path of these rays after entering the prism.

Show, with the help of a diagram, how unpolarised sunlight gets polarised due to scattering.

Two polaroids P₁ and P₂ are placed with their pass axes perpendicular to each other. An unpolarised light of intensity I_o is incident on P₁. A third polaroid P₃ is kept in between P₁ and P₂ such that its pass axis makes an angle of 45° with that of P₁. Determine the intensity of light transmitted through P₁, P₂ and P₃