Thin Lenses Zahra Pirvali University Senior College Dr Shahraam Afshar University of Adelaide Centre of Expertise in Photonics.

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Thin Lenses Zahra Pirvali University Senior College Dr Shahraam Afshar University of Adelaide Centre of Expertise in Photonics

“This material has been developed as a part of the Australian School Innovation in Science, Technology and Mathematics Project funded by the Australian Government Department of Education, Science and Training as a part of the Boosting Innovation in Science, Technology and Mathematics Teaching (BISTMT) Programme.”

THIN LENSES

A lens is a transparent material made of glass or plastic that refracts light rays and focuses (or appear to focus) them at a point.

Two major types: Convex or converging lens: Thick in the centre Concave or diverging lens: Thin in the centre

Important terms: Optical centre Principal axis Focal point Focal length

Optical centre: The geometric centre of a lens. Light rays passing through this point do not refract. Optical centre

Principal axis: A line that passes through the optical centre and makes 90° angle with the plane of lens. Principal axis

Focal point: As parallel light rays pass through a lens, they all focus at a point called focal point. f : focal point

Focal length: The distance between the optical centre and the focal point. focal length

The thicker the lens, the shorter the focal length.

There is no real focal point in a concave lens. However it appears that the parallel rays come from a point, called a virtual focal point. Virtual focal point

How to find the focal length of a convex lens: Place a ‘distant’ object preferably a candle in front of the lens. Let the light rays (which will be parallel) pass through the lens. Use a piece of paper as a screen on the other side of the lens and move it back and forth to find the sharpest image of the candle flame. Measure the distance between the screen and the centre of the lens to determine the focal length. Note: the parallel light rays focus to create an image.

Images Lenses can create images of objects with different nature. Depending on where an object is located relative to a lens, the image may be different in terms of its size, direction, reality and the distance from lens.

Ray Diagrams By tracing the rays through a lens in a ray diagram we can determine the nature of an image.

1.Show the lens as a vertical line. 2.Draw the principal axis (at 90° to the lens) and use it as a base line. 3.Draw the object as an upright arrow on the principal axis at a distance from lens. 4.Start the light rays from the tip of the arrow (object) and pass them through the lens. 5.The image of the tip is formed where the passing rays meet each other. 6.Join this point to the base line and complete the image.

Note: Use solid lines to show real rays and dashed lines to converge the real lines on the OTHER SIDE of the lens. This makes it easier to realize the real/virtual images.

The three most useful rays Ray 1: goes parallel to the principal axis and then refracts through focal point. F 1

Ray 2: goes straight through the optical centre without any refraction. 2

Ray 3: goes through the focal point, which is located on the same side as object is, then refracts parallel to the principal axis. F 3

F F 1 2 3

Image Description Comment on the following to describe the image: Real/virtual Upright/inverted Enlarged/diminished Further/closer to the lens than of object

Real image The actual rays of light (solid lines in a diagram) create the image. It can be projected on a screen.

Virtual image The real rays of light appear to form the image. While they diverge on one side of the lens, they can be continued (dashed lines in a diagram) to converge on the other side to form the image. They can be seen through the lens but not on a screen.

Describe the image of the following positions of the object by ray tracing : Beyond 2F, at 2F, between F and 2F, at F, between F and lens, at a far distance.

Beyond 2F 2F2F 2F F F Image is: Real Upright Diminished Closer to the lens

At 2F 2F2F 2F F F Image is: Real Inverted Same size Same distance from the lens

Between F and 2F 2F2F 2F F F Image is: Real Inverted Enlarged Further from the lens

At F 2F2F 2F F F Image is: At infinity

Between F and lens 2F2F 2F F F Image is: virtual Upright Enlarged Farther from the lens Magnifying glass

At a far distance from the lens 2F2F 2F F F Image is: Real Inverted Diminished At the focal point

Image in a concave lens F F Image is: Virtual Up right Diminished Closer to the lens

Optics Lab Centre of Expertise in Photonics The University of Adelaide

Murray: please add the video clip here