Optics and waves: light, reflection, refraction, lenses and sound
What this domain tests
Waves and optics covers wave quantities, sound as a mechanical wave, light as rays and as a wave, reflection, refraction, lenses and image formation. The exam asks for calculations with v = λf, ray diagrams, the mirror and refraction rules, and practical reasoning about eyes, spectacles, echoes and optical instruments.
Core rules
- Wave quantities: v = λf and T = 1/f. Amplitude corresponds to loudness or brightness; frequency corresponds to pitch or colour.
- Sound needs a medium. In air v ≈ 340 m/s; it travels faster in water and steel. Humans hear 20 Hz to 20 kHz; above that is ultrasound.
- Light travels in straight lines in a uniform medium, at 3 × 10⁸ m/s in vacuum. Shadows and eclipses follow from this.
- Reflection: the angle of incidence equals the angle of reflection, both measured from the normal. A plane-mirror image is virtual, the same size, and as far behind the mirror as the object is in front.
- Refraction: light bends because its speed changes. Entering a denser medium it slows and bends towards the normal; leaving it bends away. Beyond the critical angle total internal reflection occurs (optical fibres).
- Lenses: convex lenses converge, concave lenses diverge. The lens equation is 1/f = 1/a + 1/b and magnification m = b/a. A convex lens gives a real inverted image when a > f, and a virtual upright one when the object is inside the focal length (magnifying glass).
- Dispersion: white light splits into colours because the refractive index depends on wavelength.
Worked example
An object stands 30 cm from a convex lens of focal length 10 cm. Where is the image, and what is it like?
1/b = 1/f − 1/a = 1/10 − 1/30 = 3/30 − 1/30 = 2/30, so b = 15 cm. Because b is positive the image is real and can be caught on a screen, and it is inverted. Magnification m = b/a = 15/30 = 0.5, so the image is half the height of the object.
Traps that cost marks
- Measuring angles from the mirror surface instead of from the normal.
- Forgetting that an echo travels to the wall and back, so distance = v·t/2.
- Mixing units, for example focal length in centimetres and object distance in metres.
- Claiming a virtual image can be projected onto a screen.
- Assuming light always bends towards the normal - that only happens when entering a denser medium.
- Drawing a ray that passes straight through the lens without bending.