Lenses and Visible Light

This section explains lenses and visible light covering, convex and concave lenses, the focal length, the magnification formula, visible light and how we see colours. 

Lenses

A lens is a transparent object, usually made of glass or plastic, that refracts (bends) light rays to change the way they travel. There are two main types of lenses:

Convex Lenses (Converging Lenses)

• Shape: Thicker in the middle, thinner at the edges.
• Effect on Light: Parallel rays of light converge (come together) at a point called the principal focus.
• Uses: Cameras, magnifying glasses, and the human eye.

Concave Lenses (Diverging Lenses)

• Shape: Thinner in the middle, thicker at the edges.
• Effect on Light: Parallel rays of light diverge (spread out) as if they were coming from a virtual principal focus behind the lens.
• Uses: Correcting short-sightedness (myopia) and some types of telescopes.

This diagram illustrates convex and concave lenses with light rays.

Image

The Focal Length

The focal length of a lens is the distance from the lens to the principal focus (the point where parallel rays converge or appear to diverge).

• A shorter focal length means a more powerful lens.
• The focal point depends on the shape and material of the lens.

Magnification Formula

Magnification describes how much larger or smaller an image is compared to the original object. The formula is:

$$\text{Magnification} = \frac{\text{Image height}}{\text{Object height}}$$ 

• If magnification > 1, the image is larger than the object.
• If magnification < 1, the image is smaller than the object.
• If magnification is negative, the image is inverted (upside down).

Example Calculation

An object is 4 cm tall and produces an image 12 cm tall. The magnification is:

$$\text{Magnification} = \frac{12}{4} = 3$$

This means the image is three times larger than the object.

Visible Light

Visible light is the portion of the electromagnetic spectrum that can be detected by the human eye. It consists of different colours, each with a specific wavelength and frequency.

Colours of Visible Light

The visible spectrum consists of:

Image

• Red (longest wavelength, lowest frequency)
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet (shortest wavelength, highest frequency)

How We See Colours

Objects reflect certain wavelengths of light while absorbing others.

• A red object reflects red light and absorbs all other colours.
• A white object reflects all colours of light.
• A black object absorbs all colours and reflects none.

This guide summarises the key concepts of lenses and visible light for GCSE Physics.