Light and Reflection: KS3 Physics Explained

Light is a form of energy that travels in straight lines called rays and can be reflected, refracted, and absorbed. Reflection is when light bounces off a surface. The law of reflection states that the angle of incidence equals the angle of reflection. Light is a core topic in KS3 physics, usually taught in Year 8 or Year 9.

How does light travel?

Light travels in straight lines called rays. This is demonstrated by the fact that shadows have sharp edges — if light bent around objects, shadows would be fuzzy. Light travels at approximately 300,000 km per second (3 × 10⁸ m/s) in a vacuum — the fastest speed possible in the universe.

Light is part of the electromagnetic spectrum — a family of transverse waves that travel at the same speed in a vacuum and do not need a medium (substance) to travel through. The electromagnetic spectrum, from longest wavelength to shortest, includes: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Visible light — the only part we can see — is itself a mixture of colours, each with a slightly different wavelength: red (longest wavelength), orange, yellow, green, blue, indigo, violet (shortest wavelength). This is why white light splits into a spectrum when it passes through a glass prism.

What are luminous and non-luminous objects?

A luminous object produces its own light. Examples: the Sun, a candle flame, an electric light bulb, a glow-worm.

A non-luminous object does not produce light — we see it because it reflects light from another source. Examples: the Moon, a book, a person.

The Moon appears bright in the night sky because it reflects sunlight. This is a common misconception to correct at KS3: the Moon does not produce its own light.

What is reflection?

Reflection occurs when light strikes a surface and bounces back. All surfaces reflect some light — that is why we can see them — but different surfaces reflect different amounts and in different ways.

Types of reflection

Specular (regular) reflection occurs on smooth, shiny surfaces such as a flat mirror. Rays that hit the surface parallel to each other are reflected parallel to each other, producing a clear image.

Diffuse reflection occurs on rough, matt surfaces such as paper or painted walls. Rays hit the uneven surface and are reflected in many different directions. We can see the object, but no image is formed.

The law of reflection

The law of reflection states:

The angle of incidence equals the angle of reflection.

Both angles are measured from the normal — an imaginary line drawn perpendicular (at 90°) to the mirror's surface at the point where the light ray meets it.

Diagram in words

Imagine a flat mirror lying horizontally. A ray of light strikes it at a point P. Draw a dotted line straight upward from P — this is the normal.

The incident ray (the ray coming in) makes an angle of 30° with the normal. This is the angle of incidence (i).

The reflected ray (the ray bouncing off) makes an angle of 30° with the normal on the other side. This is the angle of reflection (r).

Since i = r = 30°, the law of reflection is satisfied.

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

A worked example: using a ray box and plane mirror

This is a standard Year 8 practical experiment in KS3 physics.

Equipment: ray box, plane mirror, protractor, paper.

Method:

Place the mirror on paper and draw a line along its reflecting surface.

Draw a normal (perpendicular line) at the midpoint.

Shine the ray box at an angle of 30° to the normal.

Mark the incident ray and reflected ray on the paper with pencil.

Measure the angle of incidence and the angle of reflection.

Repeat for angles of 45° and 60°.

Results:

Angle of incidence (i) | Angle of reflection (r)
30° | 30°
45° | 45°
60° | 60°

Conclusion: In every case, i = r. The law of reflection holds for a flat (plane) mirror.

Images in a plane mirror

When you look in a flat mirror, you see a virtual image — an image that appears to be behind the mirror but does not actually exist there. Light does not really come from behind the mirror; your brain follows the reflected rays back to where they appear to originate.

Characteristics of an image in a plane mirror:

The image appears to be the same distance behind the mirror as the object is in front of it.

The image is the same size as the object.

The image is laterally inverted — left and right are swapped (your right hand appears as the left hand in the mirror).

The image is upright (not upside down).

The image is virtual (cannot be projected onto a screen).

How is reflection different from refraction?

At KS3, reflection and refraction are often confused:

Reflection — light bounces off a surface. The light stays in the same medium (e.g., air → reflects off mirror → stays in air). No change in speed.

Refraction — light passes through from one transparent medium into another (e.g., air into glass or water). The change in speed causes the ray to bend. This is why a straw appears bent in a glass of water.

Light and the law of reflection in real life

Periscopes use two mirrors angled at 45° to allow someone to see over obstacles (e.g., in a submarine).

Rear-view mirrors in cars use flat mirrors — the driver sees a laterally inverted image of what is behind.

Dentist's mirror is a small curved (concave) mirror that reflects and magnifies the view inside the mouth.

Reflective road markings and cat's eyes use retroreflectors — small glass beads or prisms that reflect headlight beams back toward the driver.

According to the Department for Education's Science Programmes of Study for Key Stage 3 (published 2013, updated 2014), pupils should be taught about the properties of light, including reflection, and the law of reflection using ray diagrams. BBC Bitesize KS3 Physics describes the law of reflection and image formation in plane mirrors as key content for Year 8.

Frequently asked questions

What is the law of reflection?

The law of reflection states that the angle of incidence equals the angle of reflection. Both angles are measured from the normal — the line drawn perpendicular to the surface at the point of incidence. This law applies to any smooth, reflective surface.

What is the difference between a real image and a virtual image?

A real image is formed where light rays actually meet — it can be projected onto a screen. A virtual image is formed where light rays appear to come from when traced backwards — it cannot be projected onto a screen. The image in a plane mirror is always virtual.

Why does a plane mirror produce a laterally inverted image?

When you face a mirror, light from your left side hits the mirror and is reflected back to your right eye, and vice versa. The image appears to have left and right swapped. This is called lateral inversion. Up and down are not swapped — only left and right.

What is the difference between specular and diffuse reflection?

Specular reflection occurs on smooth surfaces (such as a mirror): parallel rays reflect as parallel rays, forming a clear image. Diffuse reflection occurs on rough surfaces (such as paper): rays reflect in many directions, so no image is formed but the surface is visible from all angles.

How does a periscope work?

A periscope uses two plane mirrors, each angled at 45° to the vertical, placed one at the top and one at the bottom of a vertical tube. Light enters the top mirror, reflects downward at 90°, travels down the tube, reflects off the bottom mirror at 90°, and exits horizontally toward the viewer's eye. The two reflections together preserve the orientation of the image.

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