Problems: Properties of Light

Question 1

Observations of a distant thunderstorm are made.

(a) During a lightning flash, the average wavelength of the light emitted is 5 × 10–7 m. This light travels at 3× 108 m/s.

Calculate the average frequency of this light.

(b) The interval between the lightning flash being seen and the thunder being heard is 3.6 s. The speed of sound in air is 340 m/s.

     (i) Calculate the distance between the thunderstorm and the observer.

    (ii) Explain why the speed of light is not taken into account in this calculation.

(c) A single ray of white light from the lightning is incident on a prism as shown in Fig


Complete the path of the ray to show how a spectrum is formed on the screen. Label the colours.

Question 2

Fig. below shows a ray of light, from the top of an object PQ, passing through two glass prisms.


(a) Complete the path through the two prisms of the ray shown leaving Q.

(b) A person looking into the lower prism, at the position indicated by the eye symbol, sees an image of PQ.

State the properties of this image.

(c) Explain why there is no change in direction of the ray from P at points A, C, D and F.

(d) The speed of light as it travels from P to A is 3 × 108 m/s and the refractive index of the prism glass is 1.5.

Calculate the speed of light in the prism.

(e) Explain why the ray AB reflects through 90° at B and does not pass out of the prism at B.

Question 3

Fig. below is a drawing of a student’s attempt to show the diffraction pattern of water waves that have passed through a narrow gap in a barrier.


(a) State two things that are wrong with the wave pattern shown to the right of the barrier.

(b) Sketch the wave pattern when the gap in the barrier is made four times wider.

(c) The waves approaching the barrier have a wavelength of 1.2 cm and a frequency of 8.0 Hz.

Calculate the speed of the water waves.

(d) Define wavefront and frequency of a wave.

Question 4

Fig. below shows a ray of light OPQ passing through a semi-circular glass block.


(a) Explain why there is no change in the direction of the ray at P.

(b) State the changes, if any, that occur to the speed, wavelength and frequency of the light as it enters the glass block.

(c) At Q some of the light in ray OPQ is reflected and some is refracted.

Copy the figure and draw in the approximate positions of the reflected ray and the refracted ray.

Label these rays.

(d) The refractive index for light passing from glass to air is 0.67.

Calculate the angle of refraction of the ray that is refracted at Q into air.

Question 5

Fig. below shows the parts of the electromagnetic spectrum.


(a) Name one type of radiation that has

(i) a higher frequency than ultra-violet,

(ii) a longer wavelength than visible light.

(b) Some γ-rays emitted from a radioactive source have a speed in air of 3.0 x 108m/s and a wavelength of 1.0 x 10–12m.

Calculate the frequency of the γ-rays.

(c) State the approximate speed of infra-red waves in air.

(d) What is electromagnetic spectrum ?

(e) What is the frequency of radio waves?