Light
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
Light is a transverse electromagnetic wave that travels at 3 × 10^8 m/s in a vacuum, and obeys the same reflection, refraction and diffraction rules as other waves.
Reflection of Light
- Normal: a line drawn at right angles to a surface, at the point where a ray meets it
- Angle of incidence (i): the angle between the incident ray and the normal
- Angle of reflection (r): the angle between the reflected ray and the normal
- Law of reflection: the angle of incidence equals the angle of reflection (i = r)
Plane Mirror Images
A plane (flat) mirror forms an image with the following characteristics:
- Same size as the object
- Same distance behind the mirror as the object is in front of it
- Virtual - cannot be projected onto a screen
- Laterally inverted - left and right are swapped
- Upright - the same way up as the object
Formation of an image by a plane mirror
Extended Only
Constructing Mirror Images
- Mark the image at the same perpendicular distance behind the mirror as the object is in front of it
- To find a reflected ray's path, draw the incident ray to the mirror, construct the reflected ray so that i = r, and, if needed, extend reflected rays backward (as dashed lines) behind the mirror - the point where two extended rays cross locates the image
Worked Example: Locating a Mirror Image
Museli places an object 8 cm in front of a plane mirror. How far behind the mirror does the image form, and what are its characteristics?
- Image distance behind mirror = object distance in front of mirror = 8 cm
- The image is virtual, upright, the same size as the object, and laterally inverted
Refraction of Light
- Normal: a line drawn at right angles to the boundary between two media, at the point where a ray crosses it
- Angle of incidence (i): the angle between the incident ray and the normal, in the first medium
- Angle of refraction (r): the angle between the refracted ray and the normal, in the second medium
- Light bends toward the normal when it slows down entering an optically denser medium, and away from the normal when it speeds up leaving it; its frequency never changes
Investigating Refraction Using Glass Blocks
- Method: place a transparent block (e.g. rectangular, semicircular or triangular) on a sheet of paper and trace its outline; direct a ray of light from a ray box into the block at a measured angle of incidence; mark the ray's path entering and leaving the block, then remove the block and join the marked points with straight lines to show the complete path
- Measure the angle of incidence and angle of refraction at each boundary using a protractor, with both angles measured from the normal
- Repeating with different angles of incidence, and with differently shaped blocks, shows that the ray always bends toward the normal on entering the (optically denser) block, and away from the normal on leaving it
Investigating refraction with a glass block
Passage of Light Through a Transparent Block
- At the first boundary (air to glass), the ray bends toward the normal as it slows down entering the glass
- At the second boundary (glass to air), the ray bends away from the normal as it speeds up leaving the glass
- For a rectangular block, the emergent ray is parallel to the original incident ray, but laterally displaced (shifted sideways) from it
Passage of light through a rectangular glass block
Thin Converging Lens
Action on a Parallel Beam of Light
- A converging (convex) lens is thicker in the middle than at its edges; a parallel beam of light passing through it is brought together (converged) to a single point, the principal focus
- Rays of light from an object a long way away can be assumed to be parallel by the time they reach the lens
Action of a converging lens on a parallel beam of light
Key Terms
- Principal axis: the line passing through the centre of the lens, at right angles to the lens
- Optical centre: the point at the centre of the lens where the principal axis crosses it; a ray passing through the optical centre continues without changing direction
- Principal focus (focal point, F): the point where a parallel beam converges; there is a principal focus on each side of the lens, at equal distances from the optical centre
- Focal length (f): the distance from the optical centre to the principal focus
Ray Diagrams for a Converging Lens
A ray diagram uses two rays from the top of the object to locate the top of the image:
- a ray travelling parallel to the principal axis, which refracts through the principal focus (F) on the far side of the lens
- a ray passing through the optical centre, which continues in a straight line without changing direction
Only real images (where the two rays actually cross) are needed. An image can be described using the terms enlarged / same size / diminished and upright / inverted.
- When the object is beyond 2F, the two rays actually cross on the far side of the lens: the image is real, inverted, diminished, and forms between F and 2F
- When the object is at 2F, the image is real, inverted, the same size as the object, and forms at 2F on the far side
- When the object is between F and 2F, the image is real, inverted, magnified, and forms beyond 2F on the far side
Real images formed by a converging lens for different object positions
| Object Position | Image Position | Image Nature |
|---|---|---|
| Beyond 2F | Between F and 2F (far side) | Real, inverted, diminished |
| At 2F | At 2F (far side) | Real, inverted, same size |
| Between F and 2F | Beyond 2F (far side) | Real, inverted, magnified |
Dispersion of Light
- Dispersion is the splitting of white light into a spectrum of colours, illustrated by the refraction of white light through a glass prism
- White light is a mixture of colours; different colours have different wavelengths and are refracted by different amounts by the prism, so they separate out
The Visible Spectrum
- The traditional seven colours of the visible spectrum, in order of decreasing wavelength (and equivalently, increasing frequency), are: red, orange, yellow, green, blue, indigo, violet
- Red light has the longest wavelength and lowest frequency, and is refracted the least; violet light has the shortest wavelength and highest frequency, and is refracted the most
Dispersion of white light by a glass prism
Interactive revision notes, videos and practice questions load below.