Wave interactions — OCR GCSE Physics
Test yourself on Wave interactions with OCR GCSE practice questions.
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Wave interactions explained
When an electromagnetic wave meets a substance, one or more things happen: the substance absorbs the wave, transmits it through, refracts it as it changes speed at a boundary, or reflects it at a surface.
Read the full explanation
The key Higher-tier idea is that how strongly each of these occurs depends on the wavelength of the wave, not just on the substance. A familiar example is a glass window: it transmits visible light well, yet absorbs much of the ultraviolet beyond the visible range and blocks some infrared. A coloured filter absorbs some visible wavelengths and transmits others, which is why it looks coloured. A shiny metal surface reflects a broad range of wavelengths, while a radio aerial responds to long wavelengths.
explain how some effects are related to differences in the velocity of electromagnetic waves in different substances (Higher tier only)
Electromagnetic waves travel at different speeds in different substances. In a vacuum all electromagnetic waves travel at the same speed, about 3.0 × 10⁸ m/s, but in glass, water or plastic the speed is lower, and the size of the reduction depends on the substance and on the wavelength. When a wave crosses a boundary at an angle and its velocity changes, its direction changes: this is refraction. A larger change in velocity produces a larger change in direction. The same velocity difference also explains why different wavelengths refract by different amounts, so white light separates into a spectrum through a prism. Total internal reflection occurs when a wave travelling in a denser substance meets a boundary at more than the critical angle and cannot refract out.
use ray diagrams to illustrate reflection, refraction and the similarities and differences between convex and concave lenses (qualitative only) (to include: how the behaviour of convex and concave lenses determine how they may be used, for example, to correct vision) (content found only in the physics separate science qualification)
Ray diagrams show how light behaves at surfaces and through lenses. For reflection, draw the normal at the point of incidence and show the angle of incidence equal to the angle of reflection. For refraction, draw the normal and show the ray bending towards the normal when entering a denser medium and away when leaving it. For lenses, draw the principal axis and the lens shape. A convex lens converges parallel rays to a focal point; a concave lens diverges them so they appear to come from a focal point on the incoming side. Compare the two: both refract light and have a focal point, but convex lenses converge and concave lenses diverge. This behaviour determines use: a convex lens corrects long sight by converging light, and a concave lens corrects short sight by diverging it. Draw accurately and label angles, normals and focal points.
construct two-dimensional ray diagrams to illustrate reflection and refraction (qualitative only – equations not needed) (content found only in the physics separate science qualification)
A two-dimensional ray diagram is a scale-free sketch in the plane of the page that traces light as straight lines with arrowheads showing direction of travel. For reflection, draw the mirror surface, mark the point where the ray strikes it, draw the normal as a dashed line at 90° to the surface at that point, then draw the incident ray and the reflected ray on opposite sides of the normal so the angle of incidence equals the angle of reflection. For refraction at a boundary between two media, draw the boundary and the normal at the point of entry; a ray passing into a denser medium bends towards the normal, and a ray passing into a less dense medium bends away from the normal. Label the normal, the angles and the media.
explain how colour is related to differential absorption, transmission and reflection (to include: specular reflection and scattering) (content found only in the physics separate science qualification)
An object looks coloured because of what happens to the different wavelengths of white light when they reach it. A surface absorbs some wavelengths and reflects others; the wavelengths that reach the eye determine the perceived colour. A red apple in white light absorbs most wavelengths and reflects mainly red, so it appears red. A filter transmits selected wavelengths and absorbs the rest, so a green filter lets green through and blocks red and blue. Specular reflection occurs at a smooth surface, where parallel rays reflect in one direction and a clear image can form. Scattering occurs at a rough surface, where light is reflected in many directions, so no clear image forms and the surface looks matt. A coloured surface under light of a different colour may appear black if that light contains no wavelength the surface can reflect.
Your focus
- State the four ways a substance may interact with electromagnetic waves: absorb, transmit, refract and reflect.
- Explain that the strength of each interaction depends on the wavelength of the electromagnetic wave.
- Select the correct description of a wavelength-dependent interaction from a set of multiple-choice options.
Show all 15 objectives
- Explain how a change in the velocity of an electromagnetic wave at a boundary produces refraction.
- Relate differences in velocity between substances to the size of the change in direction and to dispersion.
- Describe total internal reflection in terms of a wave in a denser substance exceeding the critical angle.
- Draw ray diagrams to illustrate reflection and refraction.
- Draw and compare ray diagrams for convex and concave lenses.
- Explain how the behaviour of each lens type determines its use in correcting vision.
- Construct a two-dimensional ray diagram showing reflection with the incident and reflected rays on opposite sides of the normal
- Construct a two-dimensional ray diagram showing a ray bending towards the normal on entering a denser medium
- Label a ray diagram with the normal, the angles and the media so the qualitative behaviour of the light is clear
- Explain why an object appears a particular colour in terms of the wavelengths it absorbs and reflects or transmits
- Distinguish specular reflection at a smooth surface from scattering at a rough surface
- Predict and explain the colour of an object when it is lit by light of a different colour
Wave interactions exam tips
Marking Points
- Identifies absorption as the transfer of electromagnetic wave energy into a substance, often raising its temperature.
- Recognises transmission as electromagnetic waves passing through a substance rather than being absorbed or reflected.
- Describes refraction as the change in direction of electromagnetic waves when their velocity changes on entering a different substance.
- States that reflection occurs when electromagnetic waves bounce off a surface or boundary.
- Explains that the extent of absorption, transmission, refraction or reflection depends on the wavelength of the electromagnetic wave.
- Selects the option that correctly links a named process to a wavelength-dependent example, such as a filter transmitting some visible wavelengths and absorbing others.
- States that electromagnetic waves travel at different velocities in different substances, with the velocity in a vacuum being about 3.0 × 10⁸ m/s.
- Explains that a change in velocity when a wave crosses a boundary at an angle causes the wave to change direction, which is refraction.
- Links a larger difference in velocity between two substances to a larger change in the direction of the wave.
- Explains that different wavelengths change velocity by different amounts, so they refract by different amounts and separate into a spectrum.
- Describes total internal reflection as occurring when a wave in a denser substance meets a boundary beyond the critical angle and cannot refract out.
- Applies the velocity-difference idea to a named example, such as light entering glass from air and bending towards the normal because its velocity decreases.
- Draws a reflection ray diagram with the normal at the point of incidence and equal angles of incidence and reflection.
- Draws a refraction ray diagram showing the ray bending towards the normal on entering a denser medium and away on leaving it.
- Draws a convex lens ray diagram showing parallel rays converging to a focal point on the principal axis.
- Draws a concave lens ray diagram showing parallel rays diverging as if from a focal point on the incoming side.
- Compares convex and concave lenses by stating that both refract light and have a focal point but one converges and the other diverges.
- Explains how a convex lens corrects long sight by converging light and a concave lens corrects short sight by diverging it.
- Draws the normal as a dashed line perpendicular to the reflecting or refracting surface at the point where the ray meets it
- Places the incident ray and the reflected ray on opposite sides of the normal with equal angles to it
- Shows a ray entering a denser medium bending towards the normal and a ray entering a less dense medium bending away from the normal
- Adds arrowheads to every ray so the direction of light travel is unambiguous
- Labels the normal, the relevant angles and the two media so the diagram can be read without further explanation
- Explains that an object appears a given colour because it reflects or transmits that wavelength and absorbs the others
- Distinguishes absorption from reflection and transmission when accounting for the colour seen
- Describes specular reflection at a smooth surface as reflection in a single direction that can form a clear image
- Describes scattering at a rough surface as reflection in many directions that prevents a clear image forming
- Applies the idea to a case such as a red object in blue light appearing black because no red light is available to reflect
Examiner Tips
- 💡Read each option and underline the named process (absorb, transmit, refract, reflect); eliminate any option that describes the wrong process.
- 💡Check the wavelength clause: a correct option must show the effect changing with wavelength, not just with the substance.
- 💡Use a concrete example such as a coloured filter or a glass window to test whether an option makes physical sense before selecting it.
- 💡Name the two substances and state which way the velocity changes before describing the resulting direction change.
- 💡Use the phrase 'change in velocity causes a change in direction' to make the causal link explicit in each explanation.
- 💡Keep the blue-sky scattering effect separate from velocity-difference refraction; do not use it as an example here.
- 💡Always draw and label the normal before measuring or drawing angles.
- 💡Mark the focal point clearly on the principal axis for each lens diagram.
- 💡State the similarity and the difference between the lenses explicitly when the question asks for a comparison.
- 💡Sketch the surface and the normal first, then add the rays, so the geometry is fixed before any angles are drawn
- 💡State in words which medium is denser and which way the ray bends, so the qualitative reasoning is visible even if the sketch is rough
- 💡Keep the diagram two-dimensional and clearly labelled; a tidy labelled sketch earns credit more reliably than an unlabelled artistic drawing
- 💡Name the wavelengths absorbed and the wavelengths reflected or transmitted, rather than saying only that the object is that colour
- 💡Use the words specular and scattering precisely, linking each to smooth or rough surfaces and to whether a clear image forms
- 💡Check the colour of the incident light before predicting the colour seen, because the available wavelengths limit what can be reflected
Common Mistakes
- Confuses refraction with reflection; keep refraction as a change of direction caused by a change of velocity inside a new substance, and reflection as bouncing off a surface.
- Assumes a substance treats all wavelengths identically; recognise that absorption, transmission, refraction and reflection all vary with wavelength.
- Treats transmission as the wave being absorbed and re-emitted; describe transmission as the wave passing through the substance.
- Ignores the wavelength clause and picks an option that is only about the substance; check that the chosen option mentions how the effect varies with wavelength.
- Claims that all electromagnetic waves travel at the same speed in every substance; state that the speed is the same only in a vacuum and is lower in matter.
- Attributes the blue colour of the sky to differences in wave velocity between substances; recognise that this effect is scattering and is outside this velocity-difference explanation.
- Says a wave bends whenever it enters a new substance regardless of angle; explain that a change of direction at a boundary requires the wave to arrive at an angle as well as a velocity change.
- Describes total internal reflection as ordinary reflection at any surface; link it to a wave in a denser substance exceeding the critical angle.
- Draws the angle of incidence from the surface rather than the normal; correct this by measuring both angles from the normal.
- Shows a concave lens converging parallel rays; correct this by drawing the rays diverging as if from a focal point on the incoming side.
- Omits the normal in a refraction diagram; correct this by drawing the normal at the boundary and bending the ray relative to it.
- Measures angles from the surface rather than from the normal; measure both angles between the ray and the dashed normal instead
- Draws the reflected ray on the same side of the normal as the incident ray; place the two rays on opposite sides of the normal
- Shows a ray bending the wrong way at a boundary; bend the ray towards the normal on entering the denser medium and away from the normal on leaving it
- Omits arrowheads on the rays; add an arrowhead to each ray to show the direction of travel
- States that an object gives out the colour it appears; explain instead that it reflects or transmits that wavelength and absorbs the rest
- Treats specular reflection and scattering as the same process; distinguish reflection in one direction at a smooth surface from reflection in many directions at a rough surface
- Assumes a red object always looks red; explain that in light with no red component it absorbs what arrives and appears black