Refraction of Waves, Convex & Concave Lenses, and Infrared Radiation
Refraction is the change in direction of a wave when it crosses a boundary from one transparent medium into another at an angle, caused by a change in wave speed (and wavelength, while frequency stays constant). In Triple Science GCSE Physics, refraction through glass is used by convex (converging) and concave (diverging) lenses to form real or virtual images, analysed using ray diagrams and the magnification formula. This topic also encompasses the required practicals for glass block refraction and investigating infrared emission and absorption using a Leslie's cube, alongside the physics of perfect black bodies and the Earth's radiative temperature balance.
- Refraction & Wavefront Diagrams (Higher Tier)
- When a wave crosses a boundary between two materials at an angle to the normal, it changes speed and bends (refracts). If the wave slows down (e.g., light entering glass/water, or water waves entering shallow water), the wavelength decreases (frequency never changes) and the wave bends towards the normal. If the wave enters along the normal (at 90° to the boundary, i = 0°), it still slows down and its wavelength decreases, but it does not change direction.
- Convex (Converging) & Concave (Diverging) Lenses (Triple)
- A convex lens bulges outwards in the middle: it refracts parallel rays of light inwards so they converge at a point called the principal focus (focal point). The focal length (f) is the distance from the centre of the lens to the principal focus. A concave lens caves inwards in the middle: it refracts parallel rays outwards (diverges them) so they appear to come from the principal focus on the same side of the lens as the object.
- Lens Ray Diagrams & Real vs Virtual Images (Triple)
- Draw two construction rays from the top of the object: 1. A ray parallel to the principal axis, which refracts through the principal focus F (or appears to come from F for a concave lens). 2. A ray passing straight through the optical centre of the lens without bending. A concave lens always produces a virtual, upright, diminished image. A convex lens produces a real, inverted image when the object is beyond F, or a virtual, upright, magnified image when the object is closer than F (magnifying glass).
Refraction of Waves, Convex & Concave Lenses, and Infrared Radiation: Wave Equation (v = fλ) & Force Vector (F = ma) Simulator
Adjust physical variables on the sliders below to see real-time changes in wave geometry, resultant forces, and acceleration.
v = f × λ = 4 Hz × 2.5 m = 10.0 m/sTime Period: T = 1 ÷ 4 = 0.25 sKey Concepts & Exam Definitions (6)
- Refraction & Wavefront Diagrams (Higher Tier)
- When a wave crosses a boundary between two materials at an angle to the normal, it changes speed and bends (refracts). If the wave slows down (e.g., light entering glass/water, or water waves entering shallow water), the wavelength decreases (frequency never changes) and the wave bends towards the normal. If the wave enters along the normal (at 90° to the boundary, i = 0°), it still slows down and its wavelength decreases, but it does not change direction.
- Convex (Converging) & Concave (Diverging) Lenses (Triple)
- A convex lens bulges outwards in the middle: it refracts parallel rays of light inwards so they converge at a point called the principal focus (focal point). The focal length (f) is the distance from the centre of the lens to the principal focus. A concave lens caves inwards in the middle: it refracts parallel rays outwards (diverges them) so they appear to come from the principal focus on the same side of the lens as the object.
- Lens Ray Diagrams & Real vs Virtual Images (Triple)
- Draw two construction rays from the top of the object: 1. A ray parallel to the principal axis, which refracts through the principal focus F (or appears to come from F for a concave lens). 2. A ray passing straight through the optical centre of the lens without bending. A concave lens always produces a virtual, upright, diminished image. A convex lens produces a real, inverted image when the object is beyond F, or a virtual, upright, magnified image when the object is closer than F (magnifying glass).
- Magnification Formula & Visible Colour Filters (Triple)
- Magnification = Image height / Object height (or Image distance / Object distance). Magnification is a ratio and has NO units (a magnification > 1 means magnified; < 1 means diminished). Opaque objects have colour because they reflect those wavelengths of visible light and absorb all others (white reflects all wavelengths; black absorbs all). Colour filters work by transmitting only their own colour wavelength and absorbing all other colours.
- Required Practical: Infrared Emission (Leslie's Cube)
- A Leslie's cube is a hollow metal cube with four different vertical surface finishes: matt black, shiny black, matt white, and shiny silver/metallic. Fill the cube with boiling water and put the lid on so all four vertical faces are at the exact same temperature. Hold an infrared detector at a fixed distance (e.g., 10 cm) from each face in turn and record the IR intensity. Order from highest to lowest IR emission: Matt black > Shiny black > White > Shiny silver.
- Black Body Radiation & Temperature Balance (Triple)
- All bodies (objects), no matter what temperature they are at, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time (and the shorter the peak wavelength emitted). A perfect black body is an object that absorbs 100% of the radiation incident on it (reflects and transmits nothing); because a good absorber is also a good emitter, a perfect black body is the best possible emitter of radiation.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
When a light wave travels from air into a glass block at an angle, state what happens to its: (a) speed, (b) frequency, (c) wavelength, and (d) direction.
Describe the three words (Real/Virtual, Upright/Inverted, Magnified/Diminished) for the image formed by: (a) a concave lens (any object position), and (b) a convex lens used as a magnifying glass (object closer than F).
An object of height 15 mm is viewed through a magnifying glass, producing a virtual image of height 6.0 cm. Calculate the magnification.
What colour does a red T-shirt appear when viewed through a blue colour filter, and why? (Triple Science)
In the Leslie's cube required practical, why do you fill the cube with hot water rather than heating four separate metal plates over Bunsen burners?
Why are marathon runners wrapped in shiny silver foil blankets at the end of a race, while solar hot-water pipes on roofs are painted matt black?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Use a wavefront model to explain why a beam of light refracts towards the normal when it travels at an angle from air into a rectangular glass block, and state what happens if the light hits the glass block along the normal (at 90° to the surface). (Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Light travels more slowly in optically denser glass than it does in air [1 mark]. 2. When a wavefront approaches the air-glass boundary at an angle, one side of the wavefront enters the glass first and slows down, while the other side of the wavefront is still in the air and continues travelling at a faster speed [1 mark]. 3. This difference in speed across the wavefront causes the wavefront to pivot (change direction) towards the normal, and the wavelength becomes shorter [1 mark]. 4. If the wavefront hits the boundary along the normal (parallel to the surface), all parts of the wavefront enter the glass at the exact same instant and slow down together, so the light does not change direction [1 mark].
- Q2
An object of height 2.0 cm is placed 3.0 cm in front of a convex (converging) lens of focal length 6.0 cm. Explain how to construct a ray diagram to locate the image, describe the three properties of the image formed, and state what optical instrument uses a convex lens in this way. (Triple Science) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Ray 1: Draw a ray from the top of the object parallel to the principal axis to the lens, which then refracts to pass through the principal focus (F) at 6.0 cm on the far side of the lens [1 mark]. 2. Ray 2: Draw a second ray from the top of the object passing straight through the optical centre of the lens without changing direction [1 mark]. 3. Because the object (at 3.0 cm) is closer to the lens than the focal length (6.0 cm), the two refracted rays diverge; trace both rays backwards with dashed lines on the same side of the lens as the object until they intersect to locate the top of the image [1 mark]. 4. Image properties: The image is virtual, upright, and magnified (height = 4.0 cm at a distance of 6.0 cm behind the lens) [1 mark]. 5. Application: A magnifying glass [1 mark].
- Q3
Describe how a student can use a Leslie's cube, a kettle, a ruler, and an infrared detector to investigate how the surface colour and texture of an object affect the rate at which it emits infrared radiation. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Place the empty Leslie's cube (which has four vertical faces: matt black, shiny black, matt white, and shiny silver) on a heatproof mat [1 mark]. 2. Boil water in a kettle, fill the Leslie's cube with the boiling water, and place the lid on so all four vertical faces reach the exact same temperature [1 mark]. 3. Hold the infrared detector pointing horizontally at the centre of the matt black face at a fixed distance (e.g., 10 cm, measured with a ruler) and record the infrared intensity reading on the meter [1 mark]. 4. Repeat the measurement for the shiny black, matt white, and shiny silver faces, keeping the distance between the infrared detector and the face constant at 10 cm (control variable) [1 mark]. 5. Results: The matt black face gives the highest reading (best emitter) and the shiny silver face gives the lowest reading (poorest emitter) [1 mark].
- Q4
Explain in terms of absorption and emission of radiation why the average temperature of a cloudless desert drops sharply at night, and why the average global temperature of the Earth is gradually increasing due to human activity. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The temperature of any body depends on the balance between the rate at which it absorbs radiation and the rate at which it emits radiation [1 mark]. 2. At night in a desert, the ground absorbs zero solar radiation from the Sun, but continues to emit infrared radiation out into space (with no clouds to absorb and re-radiate it back down); because emission > absorption, the temperature drops rapidly [1 mark]. 3. Human activities (burning fossil fuels, deforestation) increase the concentration of greenhouse gases (CO₂ and methane) in the Earth's atmosphere, which absorb outgoing long-wavelength infrared radiation emitted by the Earth [1 mark]. 4. As a result, the Earth and its atmosphere absorb solar radiation from the Sun at a slightly faster rate than they emit infrared radiation back into space, causing the average global temperature to rise until a new equilibrium is reached [1 mark].
Common Exam Questions (4)
Which way does a light ray bend when it enters and leaves a glass block, and how do wavefronts explain this?
Remember 'fast': Faster = Away from the normal; Slower = Towards the normal. When light travels from air into denser glass at an angle, it slows down: the side of the wavefront that hits the glass first slows down first while the other side keeps moving fast in air, causing the wavefronts to pivot and bend towards the normal (so angle of refraction r < angle of incidence i), and wavelength decreases. Leaving glass into air, it speeds up and bends away from the normal.
What is the difference between a real image and a virtual image in GCSE Physics?
A real image is formed where light rays actually converge (cross) after passing through a lens; a real image can be projected onto a screen (e.g., on the retina of the eye or a cinema screen) and is always inverted (upside down). A virtual image is formed where light rays only appear to come from when traced backwards with dashed lines; a virtual image cannot be projected onto a screen (e.g., a magnifying glass or a concave lens) and is always upright.
Which type of surface is the best emitter and absorber of infrared radiation, and which is the worst?
Matt black surfaces are the best emitters and best absorbers of infrared radiation (and worst reflectors). Shiny metallic / silvered surfaces are the worst emitters and worst absorbers of infrared radiation (and best reflectors). In the Leslie's cube required practical, all four vertical faces are at the exact same temperature (heated by the boiling water inside), yet the infrared detector records the highest IR intensity from the matt black face.
What happens to the temperature of an object if it absorbs radiation faster than it emits radiation?
For any body (including a planet like the Earth) to stay at a constant temperature, it must emit radiation at the exact same average rate that it absorbs radiation. If an object absorbs infrared/electromagnetic radiation at a faster rate than it emits radiation, its temperature increases.
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