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Light - Lenses 02
What is the focal length of a lens which has a power of 0.5? Find out in this quiz.

Light - Lenses 02

In GCSE Physics you learn how lenses bend light, form images and help us focus. This quiz checks your skills with ray diagrams and lens rules.

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Fascinating Fact:

Ray diagrams for lenses usually show three key rays: one through the centre, one parallel to the axis, and one through the focus, to locate the image.

In GCSE Physics, you learn how converging and diverging lenses bend light, form real and virtual images, and help us design devices such as cameras, magnifying glasses and corrective spectacles.

  • Principal axis: The straight line that passes through the centre of a lens and its focal points.
  • Focal length: The distance from the centre of a lens to its principal focus.
  • Real image: An image formed where light rays actually meet and can be projected onto a screen.
How do I draw a ray diagram for a convex lens in GCSE Physics?

To draw a convex lens ray diagram, use a ray parallel to the principal axis through the focus, a ray through the centre of the lens, and a third ray if needed. Their intersection shows the image position.

What is the difference between converging and diverging lenses?

A converging lens (convex) brings parallel rays together at a focus and can form real images. A diverging lens (concave) spreads rays apart and forms only virtual images that cannot be projected onto a screen.

Why do we learn lens ray diagrams for GCSE Physics exams?

Lens ray diagrams are tested at GCSE because they help you predict image position, size and type. Understanding them supports questions on cameras, the eye, spectacles and optical instruments in the exam.

1 .
In the eye, which of the following parts does light NOT travel through?
The pupil
The lens
The cornea
The iris
The iris is the coloured part of the eye which controls the size of the pupil. So allowing more or less light to enter the eye depending on the conditions
2 .
Calculate the power of a converging lens of focal length 0.05 m.
20
-20
-2
2
The power for a converging lens is given a positive value and for a diverging lens it is given a negative value. A capital letter D is often used to signify dioptres but is not an official SI unit symbol
3 .
What is the focal length of a lens which has a power of 0.5?
2 cm
2 m
0.2 m
0.5 m
This needs you to rearrange the lens power equation
4 .
What part of the eye focuses the light?
The lens
The iris
Suspensory ligament
Retina
The iris limits the amount of light entering the eye, the suspensory ligament supports the eye and the retina is the 'screen'
5 .
The power of a lens (SI system) can be calculated using which of the following formulae?
Power = 1 / focal length (in m)
Power = focal length / 1
Power = 1 / focal length (in cm)
Focal length (m) = 1 / Power2
Remember to convert focal length to metres
6 .
What is the power of lens which has a focal point of 20 cm?
0.05
0.5
5
50
Remember that focal length needs to be changed into metres
7 .
A lens which focuses light to a point is called a what?
A converging lens
A diverging lens
A meniscus lens
A biconvex lens
Converging means coming together
8 .
What is the power of a lens dependent on?
The type of wave incident on the lens
The focal length of the lens
The time of day
The way the lens is being held
The focal length depends on the curvature of the lens and the material from which it is made
9 .
What is chromatic aberration?
Where light is split into constituent colours as it travels through a lens
Separate rays of coloured light are converged into one beam of chromatic light
Where chromatic light is split into two colours after travelling through a lens
Chromatic aberration does not affect light
It decreases the quality of the image
10 .
What is the unit of power for a lens?
Watt
Joule
Dioptre
Metre
The higher the number of dioptres, the more a lens will bend light rays
You can find more about this topic by visiting BBC Bitesize - Lenses

Author:  Martin Moore

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