# 3.2.4 Thin Converging Lens

### Converging Lenses

• When parallel rays of light (travelling parallel to the principal axis) pass through a lens, they are brought to a focus at a point known as the principal focus

A converging lens brings parallel rays of light to a focus

• The distance of the principal focus from the lens is called the focal length, and depends on how curved the lens is

### Forming a Real Image

• Lenses can be used to form images of objects placed in front of them
• The location (and nature) of the image can be found by drawing a ray diagram:

Diagram showing the formation of a real image by a lens

1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line
2. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly towards the principal focus
3. The image is found at the point where the above two rays meet
• The above diagram shows the image that is formed when the object is placed at a distance between one focal length (f) and two focal lengths (2f) from the lens
• In this case, the image is:
• Real
• Enlarged
• Inverted
• The following diagram shows what happens when the object is more distanced – further than twice the focal length (2f) from the lens:

Diagram showing the formation of a real image by a lens with the object at distance

• In this case the image is:
• Real
• Diminished (smaller)
• Inverted
• If the object is placed at exactly twice the focal length (2f) from the lens:

Diagram showing the formation of a real image with the object at 2f

• In this case the image is:
• Real
• Same size as the object
• Inverted
Extended Only

### Features of a Real Image

• A real image is one formed by the convergence of rays of light
• A real image can be projected onto a screen
Extended Only

### Magnifying Glasses

• If the object is placed closer to the lens than the focal length, the emerging rays diverge and a real image is no longer formed
• When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left. This point can be found by extending the rays backwards (creating virtual rays)
• A virtual image will be seen at the point where these virtual rays cross

A virtual image is formed by the divergence of rays from a point

• In this case the image is:
• Virtual
• Enlarged
• Upright
• Using a lens in this way allows it to be used as a magnifying glass
• When using a magnifying glass, the lens should always be held close to the object

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
Close