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

Quick recall for OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), topic P5.3 Wave interactions. For full explanations and worked examples, read the study guide. P5.3a and P5.3b are on both tiers. P5.3c, P5.3d and P5.3e are Higher Tier only.

Links: OCR GCSE Physics hub, printable checklist, practice questions, free diagnostics.

P5.3a: four outcomes at a boundary

Outcome	Meaning	Example
Absorbed	Energy passes to the material	Water absorbs microwaves; skin absorbs infrared
Transmitted	Wave passes through	Window glass transmits visible light
Reflected	Wave bounces off	Metal reflects radio waves and microwaves
Refracted	Direction changes as speed changes	Light entering a glass block

What happens depends on the substance and the wavelength. Always name both in an answer.

P5.3b: velocity and refraction

Key facts:

• Speed in a vacuum is about 3.00 × 10⁸ m/s. It is lower in glass, perspex and water.
• Crossing a boundary: frequency constant, speed changes, wavelength changes.
• Equation to use: v = f λ, with v in m/s, f in Hz and λ in m.
• Entering a medium where the wave is slower, the ray turns towards the normal; entering one where it is faster, it turns away from it.
• Along the normal: no change of direction.

Explaining the bend in steps:

1. One end of the wavefront enters the new substance first.
2. That end changes speed while the other end is still moving at the old speed.
3. The wavefront turns, so the direction changes.

Method for a wavelength problem:

1. Convert nm to m (× 10⁻⁹).
2. Write v = f λ and rearrange.
3. Keep the frequency fixed when the substance changes.
4. Give units and a sensible number of significant figures.

Worked reminder: infrared with frequency 3.0 × 10¹⁴ Hz has wavelength 1.0 × 10⁻⁶ m in air (3.00 × 10⁸ ÷ 3.0 × 10¹⁴). In water, with speed 2.25 × 10⁸ m/s, it is 7.5 × 10⁻⁷ m.

P5.3d: ray diagram rules (Higher Tier only)

Situation	Rule
Reflection	The two angles are equal, each taken between the ray and the normal
Into a slower medium	Ray bends towards the normal
Into a faster medium	Ray bends away from the normal
Through a parallel-sided block	Emergent ray is parallel to the incident ray, shifted sideways
Along the normal	No bend

Checklist for any ray diagram: straight lines with a ruler, arrows on rays, dashed normal at every boundary, angles labelled, nothing measured from the surface.

Equations are not needed for this statement.

P5.3c: lenses (Higher Tier only)

Feature	Convex	Concave
Shape	Thicker in the middle	Thinner in the middle
Effect on parallel rays	Converge to the principal focus	Diverge
Real image possible?	Yes	No
Typical image	Depends on object distance	Virtual, upright, smaller
Vision use	Long sight	Short sight

Three construction rays for a convex lens:

1. Parallel to the axis, then through the principal focus.
2. Through the centre of the lens, undeviated.
3. Through the principal focus, then parallel to the axis.

Convex lens images:

Object distance	Image
Beyond 2F	Real, inverted, smaller
Between F and 2F	Real, inverted, larger
Inside F	Virtual, upright, larger

Similarities between lenses: both refract at two surfaces, both have principal foci, and a centre ray goes straight on. Qualitative work only.

P5.3e: colour (Higher Tier only)

• Primary colours of light: red, green, blue.
• Secondary: red + green = yellow; green + blue = cyan; red + blue = magenta.
• A filter transmits its own colour and absorbs the others. It does not add colour.
• An opaque object shows the colour it reflects. It absorbs the rest.
• White reflects all visible colours; black absorbs nearly all.
• Specular reflection: smooth surface, parallel rays stay parallel, you see an image.
• Scattering: rough surface, rays leave in many directions, so you can see the surface from any angle.

Method for a colour question:

1. List the colours in the incoming light.
2. For a filter, list what it transmits.
3. For an object, list what it reflects.
4. Only colours present in the light and also reflected or transmitted reach the eye.
5. If none are left, the answer is black (no light).

Worked reminders

Wavelength change (both tiers). Green light of frequency 5.5 × 10¹⁴ Hz is in air (3.00 × 10⁸ m/s) and then in water (2.25 × 10⁸ m/s).

Air:    λ = 3.00 × 10⁸ ÷ 5.5 × 10¹⁴ = 5.45 × 10⁻⁷ m  (545 nm)
Water:  λ = 2.25 × 10⁸ ÷ 5.5 × 10¹⁴ = 4.09 × 10⁻⁷ m  (409 nm)

The frequency is the same in both. The wavelength is shorter in water because the speed is lower.

Reading a speed change (both tiers). If light’s speed falls from 3.00 × 10⁸ m/s to 2.0 × 10⁸ m/s, it has lost one third of its speed. Its wavelength falls by the same fraction.

Colour chain (Higher Tier only). White light passes through a cyan filter, then a yellow filter.

1. Cyan transmits green and blue, and absorbs red.
2. Yellow transmits red and green, and absorbs blue.
3. Only green is transmitted by both. The light that emerges is green.

Colour of an object (Higher Tier only). A green leaf in white light reflects green and absorbs red and blue. In red light it has nothing to reflect, so it looks black.

Choosing the lens (Higher Tier only). Rays from a distant object come to a focus in front of the retina. The eye is too strong for its length. Fit a concave lens so the rays diverge slightly before they enter the eye.

Practical links

The specification suggests two lens investigations and a filter investigation. Know what each one shows:

• Lenses of both types and rays of light: concave lenses spread the rays and convex lenses bring them together.
• A convex lens and a bulb: move the bulb nearer to or further from the lens and record where a sharp image forms on a screen. The image size and position change, and inside the focal length no image forms on the screen.
• Coloured filters with light sources: record which colours pass through each filter, alone and in pairs. Filters take colours away. They do not add any.

For a method or evaluation question, say what you change, what you measure, and one control, such as keeping the room dark so stray light does not blur the image.

Must-know distinctions

• Normal (90° to the surface) versus the surface itself.
• Frequency (fixed by the source) versus wavelength (depends on the substance).
• Convex (converges) versus concave (diverges).
• Real image (rays meet, can be on a screen) versus virtual image (rays only appear to come from it).
• Specular reflection versus scattering.
• Filter (removes colours) versus source (supplies colours).

Quick self-test

1. Name the four things that can happen to an electromagnetic wave at a boundary.
2. Which quantity stays the same when light enters glass from air?
3. A ray goes from air into water. Does it bend towards or away from the normal?
4. A wave has frequency 9.0 × 10⁷ Hz. Find its wavelength in air, using 3.00 × 10⁸ m/s.
5. Light of frequency 5.0 × 10¹⁴ Hz has wavelength 380 nm in a plastic. Find its speed there.
6. A ray hits a plane mirror at 40° to the normal. What is the angle between the reflected ray and the mirror?
7. (Higher Tier only) Which lens can form a real image on a screen?
8. (Higher Tier only) Describe the image of an object placed inside the focal length of a convex lens.
9. (Higher Tier only) What colour does a yellow filter transmit from white light, in terms of primary colours?
10. (Higher Tier only) A blue shirt is lit by pure red light. What colour does it look, and why?
11. (Higher Tier only) Why can you see a sheet of paper from every angle but only see your face in a mirror from some positions?

Answers

1. Absorbed, transmitted, reflected, refracted.
2. Frequency.
3. Towards the normal, because the light slows down.
4. λ = 3.00 × 10⁸ ÷ 9.0 × 10⁷ = 3.3 m (3.33 m before rounding).
5. v = 5.0 × 10¹⁴ × 380 × 10⁻⁹ = 1.9 × 10⁸ m/s.
6. The reflected ray is at 40° to the normal, so 90° − 40° = 50° to the mirror.
7. A convex lens.
8. Virtual, upright and larger. The lens acts as a magnifying glass.
9. Red and green, which combine to look yellow. It absorbs blue.
10. Black or very dark. The shirt reflects blue but absorbs red, and there is no blue in the light.
11. Paper scatters light in many directions. A mirror gives specular reflection, so parallel rays stay parallel and only reach certain positions.

Where marks are usually lost

• Taking an angle from the surface line when it should be taken from the normal.
• Saying “frequency changes” in refraction.
• Describing refraction without saying the wave speed changes.
• Giving the wavelength in nm into an equation that needs m.
• Writing that a filter “gives” colour rather than absorbing the others.
• Drawing a ray diagram freehand, with unclear angles or no arrows.
• Calling any image “real” without checking whether the rays actually meet.
• Writing “the glass blocks light” instead of naming the wave type and the outcome.

Official syllabus

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