OCR GCSE Physics A: Reflection, refraction, lenses and colour (J249)

This guide is aligned to the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), and covers topic P5.3 Wave interactions, statements P5.3a to P5.3e. Statements P5.3a and P5.3b are assessed on both tiers. Statements P5.3c, P5.3d and P5.3e are in bold in the specification, so they are Higher Tier only, and each is labelled that way below.

Use it with the revision notes and the practice questions. Course pages: the OCR GCSE Physics hub and the printable checklist. To find weak spots first, try the free diagnostics.

What this topic covers

Statement	What you must be able to do	Tier
P5.3a	Recall that substances absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength	Both
P5.3b	Explain how some effects relate to differences in the velocity of electromagnetic waves in different substances	Both
P5.3c	Use ray diagrams to illustrate reflection and refraction, and the similarities and differences between convex and concave lenses (qualitative only)	Higher Tier only
P5.3d	Construct two-dimensional ray diagrams to illustrate reflection and refraction (qualitative only, equations not needed)	Higher Tier only
P5.3e	Explain how colour relates to differential absorption, transmission and reflection, including specular reflection and scattering	Higher Tier only

P5.3a: what happens when a wave meets a substance

When an electromagnetic wave reaches a boundary or travels through a material, four things can happen:

• Absorbed: the wave’s energy is transferred to the material, usually warming it.
• Transmitted: the wave passes through.
• Reflected: the wave bounces off the surface.
• Refracted: the wave changes direction as it crosses into a new substance.

Often several happen at once. The mix depends on the substance and on the wavelength of the wave. The same material can treat different parts of the spectrum in very different ways.

Examples you can use:

• Clear window glass transmits visible light but absorbs much of the ultraviolet.
• Water in food absorbs microwaves, which is why they heat it. Many plastics and ceramics transmit them.
• Metal surfaces reflect radio waves and microwaves well.
• Your skin absorbs infrared, which you feel as warmth.

A good answer names the substance, names the type of wave and says what happens. “Glass blocks light” is too vague, because glass transmits visible light.

P5.3b: wave velocity and refraction

Electromagnetic waves travel fastest in a vacuum (about 3.00 × 10⁸ m/s, and almost as fast in air). In denser transparent materials such as glass, perspex or water they travel more slowly. Changing speed is the cause of refraction.

When a wave crosses into a new substance:

• The frequency stays the same, because it is set by the source.
• The speed changes.
• Since wave speed = frequency × wavelength (v = f λ), the wavelength changes in proportion to the speed.

So when light slows down in glass, its wavelength gets shorter.

Why the direction changes

Think of wavefronts. If a wave meets a boundary at an angle, one end of each wavefront reaches the new substance first and slows down while the other end is still travelling fast. The wavefront swings round, and the direction of travel bends.

• Entering a substance where the wave is slower (air to glass): the ray bends towards the normal.
• Entering a substance where the wave is faster (glass to air): the ray bends away from the normal.
• Along the normal (angle of incidence 0°): the speed changes but there is no bend.

The normal is a line at 90° to the surface at the point where the ray hits. All angles are measured from the normal, never from the surface.

Worked example 1: wavelength in glass

Red light has frequency 4.8 × 10¹⁴ Hz. Its speed in air is 3.00 × 10⁸ m/s. In a block of glass its speed is 2.0 × 10⁸ m/s. Find its wavelength in air and in the glass.

In air:    λ = v ÷ f = 3.00 × 10⁸ ÷ 4.8 × 10¹⁴
             = 6.25 × 10⁻⁷ m  = 625 nm
In glass:  λ = 2.0 × 10⁸ ÷ 4.8 × 10¹⁴
             = 4.17 × 10⁻⁷ m  = 417 nm

The frequency is unchanged at 4.8 × 10¹⁴ Hz. The wavelength fell to two-thirds of its value because the speed fell to two-thirds.

Worked example 2: finding a speed

Blue light has frequency 6.5 × 10¹⁴ Hz. In air its wavelength is 462 nm. In water its wavelength is 346 nm. Find its speed in water.

v = f × λ = 6.5 × 10¹⁴ × 346 × 10⁻⁹
  = 2.25 × 10⁸ m/s

Check: in air, 6.5 × 10¹⁴ × 462 × 10⁻⁹ = 3.00 × 10⁸ m/s, so light is slower in water, as expected.

Worked example 3: a radio-type calculation

A microwave oven uses waves of frequency 2.45 × 10⁹ Hz. Find the wavelength in air.

λ = 3.00 × 10⁸ ÷ 2.45 × 10⁹ = 0.122 m

The same equation works across the whole electromagnetic spectrum. Always convert nanometres to metres (× 10⁻⁹) before you use it.

P5.3d: ray diagrams for reflection and refraction (Higher Tier only)

You need to construct two-dimensional ray diagrams. They are qualitative: no equations are needed, but the geometry must be right.

Reflection at a plane mirror

The law of reflection says the angle of incidence equals the angle of reflection, each one taken from the normal.

Steps:

1. Draw the mirror as a straight line.
2. Draw the normal at the point of incidence, at 90° to the mirror (dashed).
3. Draw the incident ray with an arrow, hitting that point.
4. Draw the reflected ray on the other side of the normal at the same angle.
5. Label the two angles.

If a ray hits the mirror at 35° to the normal, it leaves at 35° to the normal. That is 55° to the mirror surface, which is the commonest slip when you measure from the wrong line.

Refraction at a boundary and through a block

Steps for a ray going from air into a rectangular glass block:

1. Draw the boundary and the normal at the entry point.
2. Draw the incident ray in the air.
3. Inside the glass, draw the ray bending towards the normal, so the angle is smaller than in the air.
4. At the exit face draw a new normal. The ray bends away from the normal on leaving.
5. The emergent ray is parallel to the incident ray but shifted sideways.

Reasons: the two bends are equal and opposite because the two faces are parallel. A ray that hits at 0° goes straight through, with no sideways shift.

A faint reflected ray also appears at each surface. Add one if the question asks about reflection as well as refraction.

P5.3c: convex and concave lenses (Higher Tier only)

A lens refracts light at both surfaces. Its shape decides what it does to a beam. Qualitative work only is required.

	Convex (converging) lens	Concave (diverging) lens
Shape	Thicker in the middle	Thinner in the middle
Parallel rays	Bend inwards and meet at the principal focus	Spread outwards, as if coming from a point on the incoming side
Image	Real or virtual, depending on object position	Always virtual, upright and smaller

Similarities: both refract rays at the two surfaces; both have a principal focus on each side of the lens; a ray through the centre passes straight on.

Differences: a convex lens brings rays together and a concave lens spreads them out. A convex lens can form a real image on a screen; a concave lens cannot.

Drawing rays through a convex lens

Draw the lens as a vertical line with outward arrows (for convex) or inward arrows (for concave). Mark the principal focus F on both sides. Use two or three of these rays from the top of the object:

1. A ray parallel to the axis bends through F on the far side.
2. A ray through the centre of the lens carries straight on.
3. A ray through F on the object side emerges parallel to the axis.

Where the rays cross is the top of the image. Drop a line to the axis to draw the whole image.

Object position (convex lens)	Image
Beyond twice the focal length (2F)	Real, inverted, smaller, between F and 2F
Between F and 2F	Real, inverted, larger, beyond 2F
Closer than F	Virtual, upright, larger (a magnifying glass)

For a concave lens the parallel ray spreads out as if it came from F on the object side; trace it backwards with a dashed line. The image is on the same side as the object.

Using lenses to correct vision

• Short sight: the eye focuses distant objects in front of the retina. A concave lens spreads the rays slightly first, so the image forms on the retina.
• Long sight: the eye focuses near objects behind the retina. A convex lens converges the rays slightly first, so the image forms on the retina.

The specification’s practical suggestions include investigating how a convex lens image of a bulb changes with distance. Your table above predicts the results.

P5.3e: colour (Higher Tier only)

White light is a mixture of all visible colours. Colour depends on frequency. The primary colours of light are red, green and blue. Adding two gives a secondary colour: red + green = yellow, green + blue = cyan, red + blue = magenta.

Coloured filters

A filter does not add colour. It transmits its own colour and absorbs the rest.

• A red filter in white light transmits red only.
• A magenta filter transmits red and blue and absorbs green.
• Two filters in series transmit only what both transmit. Red then green leaves nothing, so you see black.

Coloured objects

An opaque object appears the colour of the light it reflects after absorbing the rest.

• A red book in white light reflects red and absorbs other colours.
• In green light, the same book has no red to reflect. It absorbs the green and looks black.
• White objects reflect all visible colours. Black objects absorb nearly all of them.

Specular reflection and scattering

• Specular reflection happens at a smooth surface such as a mirror. Parallel rays stay parallel, so you see an image.
• Scattering (diffuse reflection) happens at a rough surface. The normal changes from point to point, so reflected rays leave in many directions. This is why you can read a page from any angle.

Ordinary coloured objects scatter their colour. A glossy surface also gives a specular highlight in the colour of the light source.

Common errors

• Taking an angle from the surface line when it should be taken from the normal.
• Saying light “speeds up” when it enters glass. It slows down.
• Writing that frequency changes in refraction. The wavelength and speed change.
• Saying a filter “adds” colour, which is the misconception the specification itself warns about.
• Drawing a concave lens ray that actually meets at a point. The rays only appear to come from a point.
• Forgetting to convert nm to m in a calculation.

Next steps

Work through the practice questions, then use the revision notes to fix the gaps.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), first assessment 2018. Topic P5.3 Wave interactions.