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Light: Reflection, Refraction and the Electromagnetic Spectrum — Answers

Answer sheet with model answers and success criteria for the Year 9 energy, light, sound & electricity worksheet.

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Success TutoringWorksheetsAnswer sheetYear 9 · Science · Energy, Light, Sound & Electricity

Light: Reflection, Refraction and the Electromagnetic Spectrum — Answers

Part A: Reflection

Angles are always measured from the normal, an imaginary line at 90° to the surface where the ray hits it. Law of reflection: the angle of incidence equals the angle of reflection.

1.A ray of light hits a plane mirror with an angle of incidence of 35°. What is the angle of reflection?

Answer: 35°

  1. Angle of reflection = angle of incidence = 35°
2.A ray strikes a mirror at 20° to the mirror surface. What is the angle of incidence?

Answer: 70°

  1. The normal is at 90° to the surface
  2. Angle of incidence = 90° − 20° = 70°
3.You stand 1.5 m in front of a plane mirror. How far are you from your image?

Answer: 3.0 m

  1. The image is as far behind the mirror as you are in front: 1.5 m
  2. 1.5 + 1.5 = 3.0 m
4.Give three features of the image formed in a plane mirror.

Answer: Any three of: the same size as the object; upright; laterally inverted (left and right swapped); the same distance behind the mirror as the object is in front; virtual (it cannot be shown on a screen).

Part B: Refraction

Refraction is the change in direction of light as it passes from one material to another, caused by a change in speed. Entering a denser material (air → glass or water) light slows down and bends towards the normal. Entering a less dense material it speeds up and bends away from the normal.

5.A ray of light passes from air into a glass block at an angle. How does it bend?
  1. ATowards the normal, because it slows down
  2. BAway from the normal, because it slows down
  3. CTowards the normal, because it speeds up
  4. DIt does not bend

Answer: Towards the normal, because it slows down

Light travels more slowly in glass than in air, so it bends towards the normal as it enters.

6.A ray leaves water and enters air. Which way does it bend, and why?

Answer: Away from the normal, because light speeds up as it moves from water into air.

7.A ray of light that enters a glass block along the normal (at 90° to the surface) does not change direction.

Answer: True

It still slows down, but because it meets the surface head on, it does not bend.

8.Explain why a swimming pool looks shallower than it really is.

Answer: Light from the bottom of the pool is refracted away from the normal as it leaves the water. Your brain assumes light travels in straight lines, so it traces the rays back to a point higher than the real bottom.

Part C: Colour and the electromagnetic spectrum

White light is a mixture of colours: red, orange, yellow, green, blue, indigo and violet. Objects look coloured because they reflect some colours and absorb the rest.

Visible light is one small part of the electromagnetic spectrum. All electromagnetic waves are transverse, can travel through a vacuum and travel at the speed of light, c = 3 × 10⁸ m/s. From longest to shortest wavelength (lowest to highest frequency and energy): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.

9.A glass prism splits white light into a spectrum. What is this called, and why does it happen?

Answer: Dispersion. Each colour of light is refracted by a slightly different amount (violet the most, red the least), so the colours spread out.

10.A red apple is viewed in a room lit only by blue light. What colour does it appear?
  1. ARed
  2. BBlue
  3. CPurple
  4. DBlack

Answer: Black

The apple reflects only red light. There is no red light to reflect, and it absorbs the blue, so it looks black.

11.Write the seven main regions of the electromagnetic spectrum in order, from longest to shortest wavelength.

Answer: Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

12.Which type of electromagnetic wave causes sunburn and can increase the risk of skin cancer?
  1. AInfrared
  2. BUltraviolet
  3. CRadio
  4. DMicrowave

Answer: Ultraviolet

Ultraviolet (UV) has enough energy to damage skin cells and their DNA, which is why sunscreen and hats are important.

13.Give one use each for X-rays and infrared radiation.

Answer: X-rays: imaging bones or checking luggage at airports. Infrared: remote controls, thermal imaging cameras, heaters, toasters or grills.

Part D: Calculations and comparison

14.A radio station broadcasts at 100 MHz (1.0 × 10⁸ Hz). Calculate the wavelength of its radio waves.

Answer: 3.0 m

  1. λ = c ÷ f
  2. 3 × 10⁸ ÷ 1.0 × 10⁸ = 3.0 m
15.Microwaves used for wireless internet have a wavelength of 0.12 m. Calculate their frequency.

Answer: 2.5 × 10⁹ Hz (2.5 GHz)

  1. f = c ÷ λ
  2. 3 × 10⁸ ÷ 0.12 = 2.5 × 10⁹ Hz
16.The Sun is about 1.5 × 10¹¹ m from Earth. How long does its light take to reach us, in seconds and in minutes?

Answer: 500 s, about 8 min 20 s

  1. time = distance ÷ speed
  2. 1.5 × 10¹¹ ÷ 3 × 10⁸ = 500 s
  3. 500 ÷ 60 = 8.3 min (8 min 20 s)
17.Compare radio waves and gamma rays. Refer to wavelength, frequency, energy, a use of each, and why gamma rays are more dangerous to living things.[4 marks]

Model answer: Both are electromagnetic waves that travel at 3 × 10⁸ m/s in a vacuum. Radio waves have the longest wavelengths (from about a centimetre to many kilometres) and the lowest frequencies and energies; they are used to carry radio and television signals. Gamma rays have the shortest wavelengths and the highest frequencies and energies; they are used to kill cancer cells and to sterilise medical equipment. Gamma rays carry so much energy that they are ionising: they can damage DNA in cells, causing mutations or cancer, whereas radio waves do not have enough energy to do this.

Success criteria:

  • Same speed and both electromagnetic
  • Correct comparison of wavelength, frequency and energy
  • A correct use of each
  • Gamma rays are ionising and damage DNA
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