Lesson 3 To understand the effect that convex and concave lenses have on light. 2. To appreciate real world applications of concave and convex lenses.

Experiment Aim To look at how convex and concave lenses refract light.

Method Draw around the convex lens. 2. Switch on the ray box. with triple slit Sheet of Paper Convex Lens Draw around the convex lens. 2. Switch on the ray box. 3. Sketch over the rays of light. 4. Repeat using a concave lens.

Results – Convex (Converging) Lens Copy! Focal Length Focus Point A thicker lens will cause the focal length (measured in metres) to decrease. Focal Length Focus Point

Results – Concave (Diverging) Lens Copy! Focal Length Virtual Focus There is no natural focal point. Therefore, the rays are extended backwards to show the virtual focus. The focal length of a CONCAVE lens is always a NEGATIVE value.

Applications of lenses Lenses are used in a range of important devices used by humans. Lens inside a PS3. Lens from inside a photocopier. Magnifying Glass

The Eye The structure of the eye is as shown below: Iris Retina Pupil Optic Nerve Cornea Lens

The Eye The Cornea The cornea refracts light as it enters the eye. It is made of hardened tissue that protects the inner parts of the eye.

The Eye The Iris/Pupil The pupil is merely an opening in the iris, through which light enters the eye. The iris controls the size of the pupil, allowing different amounts of light to enter the eye.

The Eye The Lens The lens is used to fine tune the focussing process, further refracting light as it passes through the eye. It becomes thicker and shorter to focus on nearby objects and thinner and longer to focus on far objects.

The Eye The Retina The retina has special light sensitive cells called rods and cones, which change the incoming picture into electrical impulses.

The Eye The Optic Nerve The optic nerve takes electrical signals from the retina and transfers them to the brain. The optic nerve creates a “blindspot” at the back of the eye.

Applications of lenses – The Eye Copy! Both the cornea and the lens inside the human eye act as convex lenses, causing light to refract and come to a focus. Retina Cornea Light from faraway objects. Lens In the perfect eye (emmetropia), the light will focus on the retina.

Applications of lenses – The Eye Copy! Light from nearby objects does not travel in parallel rays. Therefore, the lens would not normally be powerful enough to focus it on the retina. Cornea Cornea Retina Light from near objects. Lens Lens In order to compensate for this, the lens will change shape.

Short Sightedness Copy! Myopia, or short sightedness, affects millions of people worldwide. Sufferers can see nearby objects clearly but those far away appear blurry. Retina Cornea Lens Short sightedness is usually caused by the eye being too long, so the light focuses short of the retina.

Short Sightedness – The Cure Copy! In order to alter the eyesight of someone who is short sighted, a CONCAVE lens is used. The concave lens diverges the rays, meaning they now focus on the retina.

Long Sightedness Copy! Sufferers can see faraway objects clearly but those nearby appear blurry. Retina Cornea Lens Long sightedness is usually caused by the eye being too short, so the light focuses long of the retina.

Long Sightedness – The Cure Copy! In order to alter the eyesight of someone who is long sighted, a CONVEX lens is used (like in “reading glasses”). The convex lens converges the rays, meaning they now focus on the retina.

2000

2007 (Int. 1)