Spectrum — ICSE Class 10 Physics Important Questions

Answer: (d) violet.

Violet has the shortest wavelength and the largest refractive index in glass, so it bends (deviates) the most; red deviates the least.

Answer: (c) Radio waves.

Radio waves have the longest wavelength (and lowest frequency) of all the listed electromagnetic radiations.

Answer: (c) scattered more than red light by air molecules.

Air molecules scatter shorter wavelengths far more strongly, so scattered blue light reaches our eyes from all directions, making the sky look blue.

Answer: (b) ultraviolet and infrared.

Ultraviolet radiation kills germs, so it sterilises instruments. Infrared radiation has a long wavelength and penetrates fog/haze well, so it is used for photography in fog and at night.

Answer: (a) Near the horizon sunlight passes through a long atmospheric path; the shorter blue wavelengths are scattered out and mostly the least-scattered red light reaches the eye, so the Sun looks red. R correctly explains A.

(i) Dispersion is the splitting of white light into its seven constituent colours (VIBGYOR) when it passes through a refracting medium such as a prism, because each colour is deviated by a different amount.

(ii) (a) Longest wavelength: red. (b) Shortest wavelength: violet.

(i) Infrared radiation: used in remote controls / for heat therapy and in night-vision and fog photography (any one).

(ii) Ultraviolet radiation: used to sterilise surgical instruments and drinking water / to detect forged bank notes and signatures (any one).

Using $c=f\lambda$c=fλ:

$f=\frac{c}{\lambda}=\frac{3\times10^{8}}{1500}=2\times10^{5}\,\text{Hz}$f=c/λ=3×10⁸1500=2×10⁵ Hz

The frequency is $2\times10^{5}$2×10⁵ Hz ($200$200 kHz).

In increasing order of frequency (increasing energy):

$\text{microwaves} < \text{infrared} < \text{ultraviolet} < \text{gamma rays}$microwaves < infrared < ultraviolet < gamma rays

(Frequency increases as wavelength decreases; microwaves have the longest wavelength here and gamma rays the shortest.)

Property of gamma rays (highest frequency): they are highly penetrating and carry very high energy; they can pass through thick metal and are used to destroy cancer cells and to sterilise medical equipment (any one).

The blue colour of the daytime sky is due to the scattering of sunlight by air molecules.

On Earth: the atmosphere contains air molecules and fine particles that scatter the shorter blue wavelengths of sunlight in all directions. This scattered blue light reaches our eyes from every part of the sky, so the sky looks blue.

In space: there is no atmosphere (no air molecules), so there is nothing to scatter sunlight. In the absence of scattering, no light reaches the astronaut's eyes except directly from the Sun and stars, so the sky appears black.

(i) Red light travels fastest inside the prism, because glass has the smallest refractive index for red ($v=\dfrac{c}{n}$v=c/n, so smaller $n$n gives larger speed).

(ii) Violet light has the greatest refractive index for the prism glass.

(iii) The refractive index of the glass varies smoothly with colour, being least for red and greatest for violet. Since deviation increases with refractive index, each colour is bent by a slightly different fixed amount, red least and violet most. This produces a definite, ordered band of colours: red, orange, yellow, green, blue, indigo, violet (VIBGYOR).

(a) White light is split into its colours on entering the prism and bends again on leaving; violet is deviated most, red least.

(b) The angle of deviation is the angle between the direction of the incident ray and the direction of the emergent ray after refraction through the prism.

(c) Two conditions for a pure spectrum: (i) the incident beam should be narrow and parallel (obtained using a slit and a converging lens); (ii) the prism should be placed in the position of minimum deviation, and a second lens should focus each colour to a sharp separate line on the screen.

(a) The electromagnetic spectrum is the orderly arrangement of all electromagnetic radiations according to their wavelength (or frequency). These waves need no material medium and are produced by accelerating charges.

(b) In order of increasing wavelength (decreasing frequency):

$\text{gamma} < \text{X-rays} < \text{ultraviolet} < \text{visible} < \text{infrared} < \text{microwaves} < \text{radio}$gamma < X-rays < ultraviolet < visible < infrared < microwaves < radio

(c) Radio waves: source - oscillating electric circuits/transmitters; use - radio, television and mobile communication. Gamma rays: source - radioactive nuclei; use - treatment of cancer and sterilisation of medical instruments.

(d) All electromagnetic waves travel through vacuum with the same speed, $c=3\times10^{8}\,\text{m s}^{-1}$c=3×10⁸ m s^-1.

(i) The phenomenon is dispersion of white light.

(ii) From the base of the prism upwards the colours are violet, indigo, blue, green, yellow, orange, red. Violet is deviated most, so it lies nearest the base; red is deviated least, so it is at the top.

(iii) Violet has a shorter wavelength and a larger refractive index in glass than red, so it is deviated (bent) more.

(iv) A second identical but inverted prism recombines the separated colours. The colours merge again to give a beam of white light emerging parallel to the original direction (recombination of spectrum).