# Lenses.

## Presentation on theme: "Lenses."— Presentation transcript:

Lenses

3.2 (c) Thin converging lens
Core Describe the action of a thin converging lens on a beam of light Use the term principal focus and focal length Draw ray diagrams to illustrate the formation of a real image by a single lens

Supplement Draw ray diagrams to illustrate the formation of a virtual image by a single lens Use and describe the use of a single lens as a magnifying glass

Using refraction : lenses summary
 There are two main types of lens: Convex Concave  Convex lenses work by bending [refracting] rays of light to a principal focus.  The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].  The image formed by a convex lens is inverted [back-to-front and upside-down].  The thicker the lens, the shorter the focal length[ƒ].

ƒ F Using Refraction : lenses
 The lens refracts all the rays to a point called the principal focus [F].  The distance between the centre of the lens and F is called the focal length [].  Imagine parallel rays of light from a distant object hitting the lens.  Draw normal lines where the rays enter the air [at 90º to the surface].  Use the first refraction rule to work out the ray direction.  Work out the direction of the refracted rays using the second refraction rule. F  Draw normal lines [at 90° to the surface] for each ray.  A lens can be thought of as a series of prisms. When light enters a more dense medium [e.g. glass], it bends towards the normal. When light enters a less dense medium [e.g. air], it bends away from the normal. ƒ

What do you think happens when…
Parallel light rays strike a convex lens? They pass through the focal point of the lens. Form a parallel beam if they pass though the focal point (F). Diverging light rays? F

ƒ Using Refraction : lenses - finding 
Hold the lens in the other hand and move it closer to the screen until a clear image appears. Hold a plain white screen in one hand. Chose a distant object [to get parallel rays of light]. Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. ƒ

Refraction : lenses 1. Find the focal length [ƒ] of your lens. 2F F F
2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 3. Place the candle >2F away from the lens and move the screen until an image appears and record observations. 4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.

Magnified or diminished
Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens

O I Refraction : lenses Object >2F away 2F F
The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

O I Refraction : lenses Object at 2F 2F F
The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

O I Refraction : lenses Object between 2Fand F away 2F F
The image [ l ] is formed further than 2F away from the lens, is inverted and magnified. I

O Refraction : lenses Object at F away 2F F
The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

I O Refraction : A magnifying glass Object between F and lens 2F F
The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

Magnified or diminished
Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens between F and 2F real diminished inverted at 2F real same size inverted real magnified inverted > 2F at infinity same side as object virtual magnified erect

Refraction : lenses Magnification = Distance from lens to image
Distance from object to lens

To do All P153 Answer all questions Extended only
Draw an accurate ray diagram to show a magnifying glass where f=10cm and the object is 5cm from the lens

Learning check

Which of the following is the most dense?

 Reflection Refraction Diffraction Total internal reflection
When light changes direction as it moves from one medium to another we call this effect what? Reflection Refraction Diffraction Total internal reflection

What happens to the speed of light as it moves from air into glass?
Decreases Increases No effect Decreases and increases

 No change in direction It bends away from the normal
If a ray of light moves from air to glass parallel to the normal what happens? No change in direction It bends away from the normal It bends towards the normal It stops

If light travelling through a medium has a speed of 150 000 000 m/s
If light travelling through a medium has a speed of m/s. What is the refractive index of the medium? 2.6 0.5 2.0 1.5

Can you…… Draw ray diagrams depicting the refraction of light by lenses? Write a sentence using the terms principal focus and focal length Draw a ray diagrams to show how a lens forms an inverted image Draw ray diagrams to illustrate the formation of a virtual image by a magnifying glass

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