Edexcel GCSE Physics (1PH0) — Light and the electromagnetic spectrum
If you wear glasses or contact lenses, your prescription is measured in a number like — this number represents the power of the lens. Lens power is a measure of how strongly a lens refracts (bends) light. It is inversely proportional to the focal length, which is the distance from the centre of the lens to the principal focus.
This means that as the focal length decreases, the power of the lens increases. A powerful lens bends light very sharply, causing the rays to converge or diverge much closer to the lens.
The mathematical relationship between power and focal length is given by the equation:
Where:
To measure the focal length of a lens practically, you can focus light from a very distant object (so the incoming rays are parallel) onto a screen. The distance between the lens and the sharp image formed on the screen is the focal length.
A converging lens has a focal length of cm. Calculate its power.
Step 1: Convert the focal length from centimetres to metres.
Step 2: Substitute the value into the equation.
Step 3: Calculate the final answer and add the correct unit.
The curvature of a lens — how much its surface is bent — determines its power. A lens with a more curved surface (often described as "fatter" or "thicker" in the middle) has a higher refraction strength.
This greater refraction bends the light more strongly, resulting in a shorter focal length and a higher power. Conversely, a flatter lens refracts light less, resulting in a longer focal length and a lower power. More curved lenses are necessary to correct more severe vision defects.
Students often divide 1 by the focal length in centimetres instead of converting to metres first — you will lose marks if you fall for this "Unit Trap"!
In "show that" calculation questions, examiners require you to write out every single step clearly (conversion, formula, substitution, answer), and the unit 'D' often earns a standalone mark.
When explaining why a lens is more powerful, use specific terminology from the Edexcel mark scheme such as "more curved", "fatter", "thicker", or having a "shorter focal length".
Lens power
A measure of how strongly a lens refracts (bends) light, calculated as the reciprocal of the focal length.
Focal length
The distance from the centre of the lens to the principal focus.
Principal focus
The point where rays of light travelling parallel to the principal axis converge (for a convex lens) or appear to diverge from (for a concave lens).
Dioptres
The standard unit of lens power, represented by the symbol D, which is equivalent to inverse metres (m⁻¹).
Curvature
The degree to which the surface of a lens is bent; a more curved lens has a smaller radius of curvature.
Refraction strength
A description of how much the path of light is changed as it passes through a lens.
Put your knowledge into practice — try past paper questions for Physics
Lens power
A measure of how strongly a lens refracts (bends) light, calculated as the reciprocal of the focal length.
Focal length
The distance from the centre of the lens to the principal focus.
Principal focus
The point where rays of light travelling parallel to the principal axis converge (for a convex lens) or appear to diverge from (for a concave lens).
Dioptres
The standard unit of lens power, represented by the symbol D, which is equivalent to inverse metres (m⁻¹).
Curvature
The degree to which the surface of a lens is bent; a more curved lens has a smaller radius of curvature.
Refraction strength
A description of how much the path of light is changed as it passes through a lens.
