LightLenses

Revision (refraction)

• Refraction occurs when the light ray changes mediums. Light traveling through air and then going through water is an example of a light ray changing medium. The speed of the light ray changes when it enters a different medium. In most cases the direction of the light also changes. We say the light bends.

Revision (refraction)

• FST = Fast to Slow, Towards Normal. If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line.

Revision (refraction)

• SFA = Slow to Fast, Away From Normal. If a ray of light passes across the boundary from a material in which it travels slow into a material in which travels faster, then the light ray will bend away from the normal line.

What is a Lens?

• A lens is a transparent material, such as glass, that has either one curved surface and one flat surface or two curved surfaces.

• When light travels through lenses, refraction occurs. The light bends either outward or inward, it depends on the lens.

Why use a lens?

• As a lens alters the path of light rays, they are used to assist humans in many ways.

• Each of us have convex lenses’ in our eyes. These are to focus the light coming from objects around us, onto the back of our eye (the retina).

Types of Lenses

• Convex lens (or converging lens): Due to refraction, light rays bend as they pass into and out of the lens. Convex lenses are shaped so that the rays converge together.

• Concave lens (or diverging lens): concave lenses are shaped to spread rays apart.

Anatomy of a Lens [1]

• If a symmetrical lens is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the lens. This imaginary line is known as the principal axis.

Anatomy of a Lens [2]

• A lens also has an imaginary vertical axis which bisects the symmetrical lens into halves.

Anatomy of a Lens [3]• light rays traveling parallel to the principal axis

will either converge or diverge. If the light rays converge (convex), then they will converge to a point. This point is known as the focal point of the converging lens. If the light rays diverge (concave), then the diverging rays can be traced backwards until they intersect at a point. This intersection point is known as the focal point of a diverging lens

Anatomy of a Lens [4]• The focal point is denoted by F. Note that

each lens has two focal points - one on each side of the lens. Lenses can allow light to pass through either face, depending on where the incident rays are coming from. Subsequently, every lens has two possible focal points. A lens also has an imaginary point which we refer to as the 2F point. This is the point on the principal axis which is twice as far from the vertical axis as the focal

point is.

Image Formation

Refraction Rules for a Converging Lens: Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray which passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens.

Image Formation• Refraction Rule for a Diverging Lens:

Any incident ray traveling parallel to the principal axis of a diverging lens

will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point). Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray which passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens.

Convex lens (Ray trace diagram)

1. Pick a point on the top of the object and draw three incident rays traveling towards the lens.

Convex lens (Ray trace diagram)

2. Once these incident rays strike the lens, refract them according to the three rules of refraction for converging lenses.

Convex lens (Ray trace diagram)

3. Mark the image of the top of the object.

Convex lens (Ray trace diagram)

4. Repeat the process for the bottom of the object.

Concave Lens (Ray trace diagram)

1. Pick a point on the top of the object and draw three incident rays traveling towards the lens.

Concave Lens (Ray trace diagram)

2. Once these incident rays strike the lens, refract them according to the three rules of refraction for double concave lenses.

Concave Lens (Ray trace diagram)

3. Locate and mark the image of the top of the object.

Concave Lens (Ray trace diagram)

4. Repeat the process for the bottom of the object.

Image Location

• Convex lenses can produce real images which can be caught on a screen or a virtual image which cannot be caught on a screen.

• Concave lenses produce only virtual images

Image Location (terminology)

• Upright – The same way up as the original object.

• Inverted – the opposite way up to the original object.

• Enlarged – bigger than the original object.• Diminished – smaller than the original image.• Magnification – the image size divided by the

size of the original object

Uses - telescope

Uses - Microscopes

Student Work

• Complete the questions on pages 232 - 233