A lens is a transparent optical medium, such as glass, with one or two curved surfaces that refract light rays. Lenses are broadly classified into convex lenses (converging lenses) and concave lenses (diverging lenses). Understanding how lenses refract light and form real or virtual images is essential in optical instruments such as cameras, magnifying glasses, and the human eye.
A side-by-side comparison of how parallel rays refract through both lens types and how the position of the object affects image properties.
A lens is a transparent optical medium, such as glass, with one or two curved surfaces that refract light rays.
Lenses bend (refract) light to form images. Convex lenses bring parallel light rays together at a focal point, making them converging lenses. Depending on how far an object is placed from a convex lens, the image can be real (can be displayed on a screen) or virtual, inverted or upright, and magnified or diminished. When an object is held closer than the focal length (\(u < F\)), a convex lens acts as a magnifying glass.
In contrast, concave lenses spread light rays outward, making them diverging lenses. Because these diverging rays never physically meet after passing through the lens, they only appear to intersect when traced backwards. Therefore, a concave lens always produces an image that is virtual, upright, diminished, and located between the object and the lens, regardless of the object distance.
Two main lens types. Convex lenses are thicker in the centre and converge light rays. Concave lenses are thinner in the centre and diverge light rays.
Lenses are classified by their shape and their effect on light rays.
Thicker in the centre than at the edges. Light rays converge at a focal point after passing through a convex lens.
Types: Biconvex, Planoconvex, Convex meniscus.
Thinner in the centre than at the edges. Light rays diverge after passing through a concave lens.
Types: Biconcave, Planoconcave, Concave meniscus.
Activity 7.2: Using a distant object to estimate the focal length.
Concept: Light rays originating from a distant object are parallel to each other.
Apparatus & Materials: Convex lens, lens holder, white screen, metre rule.
1. Position the convex lens towards a distant object (e.g., seen through an open laboratory window).
2. Adjust the position of the white screen behind the lens until a sharp image of the distant object is formed on the screen.
3. Measure the distance between the centre of the convex lens and the screen using a metre rule. This distance is the focal length (\(f\)).
Characteristics of the image formed on the screen: Real, inverted, and diminished (formed at \(F\)).
Concave Lens Limitation: The focal length of a concave lens cannot be determined using this method because concave lenses produce virtual images that cannot be captured on a screen.
Two standard principal rays are used to locate the image formed by a lens.
Convex lens: A light ray parallel to the principal axis refracts and passes through the focal point, \(F\), on the other side.
Concave lens: A light ray parallel to the principal axis refracts and appears to diverge from the focal point, \(F\), on the same side.
For both convex and concave lenses, a light ray heading towards the optical centre continues in a straight line through the optical centre without refracting.
The position and characteristics of the image depend on the object distance.
For any object distance (e.g., \(u > 2F\) or between \(F\) and optical centre):
Position of image: Between the optical centre and focal point (\(F\)), positioned between the object and the concave lens.
Characteristics: Always virtual, upright, and diminished.
Answer in your mind, then press “Check Answer”.
What type of lens is present in the human eye, and does it act as a converging or diverging lens?
Why cannot the focal length of a concave lens be determined using a distant object and a white screen like in Activity 7.2?
What must the position of the object be for a convex lens to function as a magnifying glass, and what are the characteristics of the resulting image?
All lenses form images on a screen.
Only real images can be formed on a screen (formed by convex lenses when \(u > f\)). Virtual images (such as all images formed by concave lenses and convex lenses with \(u \le f\)) cannot be captured on a screen.
Light rays passing through the centre of the lens bend because the glass is thickest there.
Light rays that pass directly through the optical centre (\(O\)) pass straight through without bending (no refraction).
Moving an object closer to a concave lens will eventually make the image real and inverted.
For any object distance, a concave lens consistently forms a virtual, upright, and diminished image.
1 What happens to parallel light rays after passing through a concave lens?
2 An object is placed at a distance of \(2F\) in front of a convex lens. What are the characteristics of the image formed?
3 The point at the centre of a lens through which light rays pass without refracting is called the .
4 To use a convex lens as a magnifying glass, the object must be placed between the focal point and the .
Drag each term or condition to the description it matches. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
Write the correct term or condition next to each description.
Answer choices: Convex lens (\(u < F\)) • Concave lens (any \(u\)) • Principal axis • Focal length (\(f\))
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