Science (English Medium) कक्षा १२ - CBSE Important Questions for Physics

Answer the following question.
An optical instrument uses a lens of 100 D for the objective lens and 50 D for its eyepiece. When the tube length is kept at 20 cm, the final image is formed at infinity.
(a) Identify the optical instrument.
(b) Calculate the magnification produced by the instrument.

A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. Find the magnifying power of the telescope for viewing distant objects when

1. the telescope is in normal adjustment,
2. the final image is formed at the least distance of distinct vision.

The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm.

What will be its new focal length when placed in a medium of refractive index 1.25?

Is focal length positive or negative? What does it signify?

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.

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.

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.`

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.

Monochromatic light of wavelength 589 nm is incident from air on a water surface. What are the wavelength, frequency and speed of (a) reflected and (b) refracted light? Refractive index of water is 1.33.

Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focusses on the focal point of the lens, giving the shape of the emergent wave front.

Find the intensity at a point on a screen in Young's double slit experiment where the interfering waves have a path difference of (i) λ/6, and (ii) λ/2. 

(i) State the essential conditions for diffraction of light.
(ii) Explain diffraction of light due to a narrow single slit and the formation of pattern of fringes on the screen.
(iii) Find the relation for width of central maximum in terms of wavelength 'λ', width of slit 'a', and separation between slit and screen 'D'.
(iv) If the width of the slit is made double the original width, how does it affect the size and intensity of the central band?

How does the refractive index of a transparent medium depend on the wavelength of incident light used ?

Why cannot two independent monochromatic sources produce sustained interference pattern?
Deduce, with the help of Young's arrangement to produce interference pattern, an expression for the fringe width.