Light and Lenses Notes

Transcription

1 Light and Lenses Notes

2 Refraction The change in speed and direction of a wave Due to change in medium Must cross boundary at an angle other than 90 o, otherwise no change in direction I R (unlike reflection) Amount of bending is determined by Optical Density of the media

3 Diagrams of Refraction Less dense Incident Ray Normal θ i More dense θ r Boundary Refracted Ray

4 Diagrams of Refraction More dense Incident Ray Normal θ i Less dense θ r Boundary Refracted Ray

5 For refraction of light: the wave speed is always greatest in the least dense medium the wavelength is always greatest in the least dense medium the frequency of a wave is not altered by crossing a boundary When light passes from a more optically dense medium into a less optically dense medium, it will bend AWAY the normal. When light passes from a less optically dense medium into a more optically dense medium, it will bend TOWARDS the normal.

6 What do you think? Will a light wave travel faster or slower as it enters more dense media? Slower If the speed changes, will the frequency change? No, just the wavelength If the speed travels slower, what will happen to the wavelength? It will decrease What is the maximum speed? The speed of light

7 Optical Density Described by Index of Refraction n stands for the index A measure of the ratio of light speed traveling in a vacuum and light speed traveling in another medium

8 Index of Refraction What is n in a vacuum? n = exactly 1 in a perfect vacuum How do you think the speed of light changes in air? Very little n = in air not significant, so use n=1 for air n > 1 for all materials As density increases, what will happen to the index of refraction? It increases

9 Substance 1 has an index of refraction of 1.5. Substance 2 has an index of refraction of 3. Which substance does light travel faster in? Substance 1 Which substance is more dense? Substance 2

10 Snell s Law n sin n sin n 1 = index of refraction in substance 1 sinθ 1 = sine of angle of incidence n 2 = index of refraction in substance 2 sinθ 2 = sine of angle of refraction MAKE SURE YOU ARE IN DEGREE MODE

11 Angles are Always measured relative to a normal Normal: Perpendicular to a surface

12 Critical Angle Occurs when light travels from a more optically dense medium into less optically dense medium air Occurs when the angle of refraction is 90 O What happens if angle of incidence is greater than the critical angle? No refraction, all light is reflected! This is referred to as Total Internal Reflection

13 Critical Angle Critical Angle

14 Applications of Critical Angle Total Internal Reflection!!! Fiber optic cables Prisms in binoculars touch screens

15 real image inverted image formed on the opposite side of lens as object formed where light rays actually converge ( cross ) visible on the screen ( projectable ) virtual image upright image formed on the same side of a lens as object formed where light rays appear to cross not visible on a screen (not projectable)

16 The two main types of lenses are convex and concave lenses. The focal length (f) of a lens depends on its shape and its index of refraction.

17 A diverging (concave) lens is thin in the center and thick at the edges. A converging (convex) lens is thick in the center and thin at the edges.

18 Concave Lenses = Diverging Lenses spread out light rays. for nearsightedness (myopia) forms virtual images only always upright and reduced aka reducing lenses

19 Convex Lenses = Converging Lenses bring light rays to a focus. for farsightedness (hyperopia) form virtual images (upright & enlarged) aka magnifying Lenses Forms real images

20 The eye contains a convex lens. This lens focuses images on the back wall of the eye known as the retina.

21 VISION PROBLEMS: MYOPIA is when image is formed in front of retina and is also known as nearsightedness and is corrected with a concave lens

22 VISION PROBLEMS: HYPEROPIA is when image is formed behind the retina and is also known as farsightedness and is corrected with a convex lens

23 VISION PROBLEMS: ASTIGMATISM is when the eye is shaped like a football rather than the normal eye that has a round shape similar to basketball. It causes certain amounts of distortion or pitched images because of the uneven bending of light rays entering the eye.

24 Parts of a Lens All lenses have a focal point (f). In a convex lens, parallel light rays all come together at a single point called the focal point. In a concave lens, parallel light rays are spread apart but if they are traced backwards, the refracted rays appear to have come from a single point called the focal point. f Real f Virtual

25 Rules for Locating Refracted Images 1. Start at top of object. Light rays that travel through the center of the lens (where the principle axis intersects the midline) are not refracted and continues along the same path. 2. Start at top of object. Light rays that travel parallel to the principle axis, strike the lens, and are refracted through the focal point (f).

26 Someone who is nearsighted can see near objects more clearly than far objects. The retina is too far from the lens and the eye muscles are unable to make the lens thin enough to compensate for this. Diverging glass lenses are used to extend the effective focal length of the eye lens.

27 Someone who is farsighted can see far objects more clearly than near objects. The retina is now too close to the lens. The lens would have to be considerable thickened to make up for this. A converging glass lens is used to shorten the effective focal length of the eye lens. Today s corrective lenses are carefully ground to help the individual eye but cruder lenses for many purposes were made for 300 years before the refractive behavior of light was fully understood.