[3 Mark Question Answer]

Question 13 Marks

(a) What are infrared rays. What is the range of wavelengths of infrared rays in $A$ ?
(b) State four important properties and four uses of infrared rays.

Answer

(a) The electromagnetic radiations beyond the red end of the visible spectrum with wavelengths from 8000 A to 1 mm $\left(10^7 A\right)$ are called infrared radiations.
(b) Properties of infrared rays:
(i) They produce heating effect.
(ii) They have very long wavelengths, hence are least scattered by fog, mist, etc.
(iii) They do not affect ordinary photographic plate.
(iv) They can easily pass through rock salt.
Uses of infrared radiations :
(i) They are used for therapeutic purposes by doctors since they produce heat.
(ii) They are used in night vision devices.
(iii) They are used to detect diseases in crops.
(iv) They are used for taking infrared photographs in foggy weather.

View full question & answer→

Question 23 Marks

(a) What is ultraviolet spectrum? What is the range of its wavelengths in $A$ ?
(b) Stale four important properties of ultraviolet rays.
(c) State four uses of ultraviolet rays.

Answer

(a) The region of spectrum which extends beyond the violet colour of the visible speetrum having wavelength range from 4000 A to 100 A is called ultraviolet spectrum.
(b) Properties of ultraviolet radiation :
(i) They produce fluorescence in substances like zinc sulphide, barium sulphide and quinine sulphate solution.
(ii) They produce chemical effects in silver salts.
(iii) They are absorbed by the atmosphere and convert oxygen to ozone.
(iv) They are electromagnetic waves travelling at a speed of $3 \times 10^8 ms^{-1}$ in air (or vacuum).
(c) Uses of ultraviolet radiation :
(i) They are used for sterilising surgical instruments and drinking water.
(ii) UV radiations when absorbed by the skin, stimulate the body to produce vitamin D.
(iii) UV radiations are used to detect adulteration in ghee.
(iv) They are used for detecting original diamonds from fake diamonds

View full question & answer→

Question 33 Marks

Briefly describe an experiment to show that a prism itself produces no colour, during disperssion of white light.

Answer

White light passing through a slit, is made to fall on an equilateral prism A , in a dark room. The dispersed light is then received on a white screen ST, having a small hole in it. The light passing out of the hole (say, green light), is made to pass through another prism B, placed in an inverted position. It is observed that the light on passing through the prism B only gets refracted, but does not suffer any dispersion.

By moving the screen ST, up or down, experiment can be conducted for any other colour of light. It is seen that in each case, only refraction takes place. (See figure given alongside.)
Thus, experiment proves that white light gets dispersed at the first face of prism only. Afterwards it gets refracted. Hence, prism does not colour the white light.

View full question & answer→

Question 43 Marks

(a) What do you understand by the term lens?
(b) What are the various kinds of lenses? Draw a neat diagram of each kind.

Answer

(a) A lens is a transparent material bounded by one or two spherical surfaces.
(b) Lenses are of two types : Converging and diverging lens.
(i) When the lens is thicker in the middle and thinner at the edges, it is called a converging or convex lens. There are three types of convex lens : (i) double convex lens. (ii) plano-convex lens, and (iii) concavo-convex lens.

(ii) When the lens is thinner in the middle and thicker at its edges, it is called concave lens or diverging lens. Concave lens is of three types: (i) double concave lens, (ii) plano-concave lens, (iii) convexo-concave lens.

View full question & answer→

Question 53 Marks

State four differences between reflection and total internal reflection.

Answer

S. No	Reflection	Total Internal Reflection
1.	Smooth polished surface is required for reflection.	No smooth, polished surface is required for total internal relflection.
2.	It takes place for all the angles of incidence.	It takes place when the rays strike at an angle greater than the critical angle.
3.	It takes place when the rays of light is incident on a plane mirror from any medium at any angle of incidence.	It takes place when the rays of light travel from a denser to a rarer medium.
4.	Some rays of light are absorbed by the reflecting material.	No rays of light are absorbed.

View full question & answer→

Question 63 Marks

Explain :
(a) Why does a stamp placed under a glass block, appear raised?
(b) Why does a fisherman aim his spear at the tail of a fish during spear fishing?
(c) Why does the sun appear bigger during sunrise or sunset?

Answer

(a) Rays of light diverging from the stamp, on emerging out of the glass slab, suffer refraction, and hence, bend away from the normal. When these refracted rays reach the eye, then they appear to come from a higher position. Thus, the stamp and the bottom of the slab appear raised.
(b) Because of refraction of light in water, the apparent position of the fish is raised up and ahead from its real position. Thus, the fisherman has to aim his spear at its tail, so that it is likely to hit on the head of the real fish. If he aims at the head of the apparent position of the fish, the spear will hit in front of the actual fish.
(c) Sun rays pass through layers of air of increasing density. During sunset or sunrise, the sunlight travels through maximum length of the atmosphere, therefore refraction is also maximum. Hence apparent position of the sun is closer to the observer than its real position. Thus it appears bigger.

View full question & answer→

Question 73 Marks

(a) Copy the diagram given alongside and clearly show the path taken by the emergent ray.
(b) (i) A ray of light passes from water to air. How does the speed of light change?
(ii) Which colour of light travels fastest in any medium except air?
(c) Name the factors affecting critical angle for a pair of media.

Answer

(a)

(b) (i) The speed of light increases when it passes from water to air.
(ii) Red colour of light travels fastest in any other medium except air
(c)(i) The refractive index of media determines its critical angle, ie., the more is the refractive index, the less is the critical angle.
(ii) The wavelength (or colour) of light determines the critical angle.

View full question & answer→

Physics [3 Mark Question Answer]

Test yourself on this topic