14.1 Reflection of Light

A free GCSE Physics lesson for AQA (8463) from OBA School: the normal, the law of reflection, images in a plane mirror, real and virtual images, specular and diffuse reflection. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Draw the normal and measure angles of incidence and reflection from it
• State and use the law of reflection
• Draw a ray diagram to show how a plane mirror forms an image, and describe the image
• Explain the difference between a real image and a virtual image
• Describe and explain specular and diffuse reflection

Key words

• Normal: A construction line drawn at 90° to a surface at the point where a ray meets it.
• Angle of incidence (i): The angle between the incident ray and the normal.
• Angle of reflection (r): The angle between the reflected ray and the normal.
• Plane mirror: A perfectly flat mirror.
• Virtual image: An image at a place where light rays only appear to come from. It cannot be shown on a screen.
• Real image: An image formed where light rays actually meet. It can be shown on a screen.
• Specular reflection: Reflection from a smooth surface: parallel rays are reflected in a single direction.
• Diffuse reflection: Reflection from a rough surface: parallel rays are scattered in many directions.

Light waves meeting a surface

Light is a transverse wave. When light meets the boundary between two materials it can be reflected (bounce back), transmitted (pass through) or absorbed (its energy is transferred to the material). Usually all three happen to some extent. What happens depends on the wavelength of the light and on the materials.

Picture a set of straight wavefronts heading towards a flat barrier at an angle. Each part of a wavefront bounces off as it arrives. Every part travels at the same speed, so the reflected wavefronts leave the barrier at the same angle as the incident ones arrived.

Key fact: Reflection changes the direction of a wave only. The reflected waves have the same speed, frequency and wavelength as the incident waves.

The law of reflection

We draw rays to show the direction light travels. To describe reflection we first draw the normal: a dashed line at 90° to the mirror where the ray hits it. Both angles are measured from the normal, not from the mirror.

Law of reflection: angle of incidence = angle of reflection (i = r)

The incident ray, the reflected ray and the normal all lie in the same flat plane.

Worked example: a torch beam on a bathroom mirror

A narrow beam hits a mirror at 55° to the mirror surface.
Angle of incidence = 90° − 55° = 35° (measured from the normal)
Angle of reflection = angle of incidence = 35°
Angle between the incident and reflected beams = 35° + 35° = 70°

Answer: i = r = 35°; the beam is turned through 180° − 70° = 110°

Exam tip: If a question gives the angle between the ray and the mirror, subtract it from 90° first.

How a plane mirror forms an image

Rays leave each point of an object in all directions. Those that hit the mirror obey the law of reflection. When the reflected rays enter your eye, your brain assumes light travels in straight lines, so you see the object at the point the rays appear to come from: the image, behind the mirror.

To draw the diagram: (1) draw two rays from the object to the mirror, (2) draw the normals and reflect each ray so that i = r, (3) extend the reflected rays backwards with dashed lines; where they cross is the image.

• virtual; Meaning no light actually reaches the place behind the mirror; it cannot be put on a screen
• upright; Meaning the same way up as the object
• same size; Meaning the image is as tall as the object
• same distance behind the mirror; Meaning as the object is in front of it, on a line at 90° to the mirror
• laterally inverted; Meaning left and right are swapped (back to front, but not upside down)

Worked example: a dance studio mirror

A dancer stands 1.5 m in front of a wall mirror.
Her image is 1.5 m behind the mirror, so it is 1.5 + 1.5 = 3.0 m from her.
She steps 0.5 m closer: she is now 1.0 m from the mirror, so her image is 2.0 m from her.
She raises her right hand: the image appears to raise its left hand (lateral inversion).

Answer: 3.0 m, then 2.0 m

Real and virtual images

• How it forms; Real image light rays actually meet (are focused) there; Virtual image light rays only appear to come from there
• On a screen?; Real image yes, it can be projected onto a screen; Virtual image no, you can only see it by looking into the mirror or lens
• Examples; Real image a cinema or classroom projector; the image on a camera sensor; Virtual image a plane mirror; a magnifying glass

Exam tip: In ray diagrams, draw virtual rays and virtual images with dashed lines, and real rays with solid lines and arrows.

Specular and diffuse reflection

Every surface obeys the law of reflection at each point. The difference is the shape of the surface:

• Specular reflection happens at a smooth surface such as a mirror, polished metal or still water. The normals all point the same way, so parallel rays stay parallel after reflection. You see a clear image.
• Diffuse reflection happens at a rough surface such as paper, a painted wall or cloth. The normals point in many directions, so parallel rays are scattered in different directions. You see the surface itself, but no image.

Worked example: a lake on a calm day and a windy day

Calm day: the water surface is flat, so reflection is specular and you see a sharp image of the hills.
Windy day: ripples make the surface uneven, so the light is scattered (diffuse) and the image breaks up.

Answer: the smoother the surface, the clearer the reflected image

Common mistakes

• Measuring the angle of incidence or reflection from the mirror surface instead of from the normal.
• Drawing the image in a plane mirror closer to the mirror than the object. It is the same distance behind as the object is in front.
• Saying the image in a plane mirror is upside down. It is upright but laterally inverted (left and right swapped).
• Drawing the extensions of rays behind the mirror as solid lines. They are virtual, so use dashed lines.
• Thinking a rough surface breaks the law of reflection. Each ray still obeys i = r; the normals just point in different directions.

Revision questions

Try these, then check your answers below each one.

Question 1. In a ray diagram, what is the normal? (Choose from: A line drawn at 90° to the surface where the ray hits it, The surface of the mirror, The path of the reflected ray, A line drawn at 45° to the mirror.)

Answer: A line drawn at 90° to the surface where the ray hits it

Working: The normal is a construction line perpendicular (at 90°) to the surface at the point of incidence. Angles of incidence and reflection are measured from it.

Question 2. A ray of light hits a plane mirror. The angle between the ray and the mirror surface is 62°. What is the angle of reflection?

Answer: 28 °

Working: Angle of incidence = 90° − 62° = 28°. Law of reflection: angle of reflection = angle of incidence = 28°.

Question 3. A ray hits a plane mirror with an angle of incidence of 35°. What is the angle between the incident ray and the reflected ray?

Answer: 70 °

Working: Angle of reflection = 35°. The angle between the rays = 35° + 35° = 70°.

Related lessons

• Previous lesson: 13.6 Required Practical: Infrared Radiation
• Next lesson: 14.2 Refraction of Light
• In this chapter: 14.3 Light and Colour
• In this chapter: 14.4 Lenses
• In this chapter: 14.5 Using Lenses

Continue in the full course

This lesson is section 14.1 (AGCPH1401) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.