Today s Topic: Refraction / Snell s Law

Transcription

1 Today s Topic: Refraction / Snell s Law Learning Goal: Students will be able to calculate the angle of reflection of a bent light wave. Take out your notes from yesterday as we learn about Snell s Law.

2 Homework Complete the Snell s Law Worksheet (Due Monday, 6/1) Complete The Law of Reflection Worksheet (Two Days Late)

3 Upcoming Test Your last test will take place on Friday, June 5 th (a week from tomorrow). Topics covered will include: Color The Law of Reflection Refraction Snell s Law Ray Diagrams (Converging & Diverging)

4 Light Bending Now that we know bends as it travels through different mediums, one might ask how much does the light bend? The amount light bends is dictated by Snell s Law. Snell s law is a formula used to describe the relationship between the angles of incidence and refraction when light refracts.

5 Snell s Law Snell s Law: n i sin θ i = n r sin θ r Where θ i = angle of incidence θ r = angle of refraction n i = refractive index of incidence medium n r = refractive index of refraction medium

6 Sample Problem A light ray traveling through air strikes a smooth, flat slab of crown glass (n = 1.52) at an angle of 30 to the normal. What is the angle of refraction?

7 Snell s Law Example Light travels from air (n=1) into an optical fiber with an index of refraction of 1.44 a) In which direction does the light bend? b) If the angle of incidence on the end of the fiber is 22, what is the angle of refraction inside the fiber?

8 Snell s Law Example a) In which direction does the light bend? Since the light is going from a low index of refraction (n=1) to high (n=1.44), the light is slowing down, so the light will bend towards normal.

9 Snell s Law Example b) If the angle of incidence on the end of the fiber is 22, what is the angle of refraction inside the fiber? n i *sin θ i = n r *sin θ r (1.00)*sin(22 ) = (1.44)*sin(θ 2 ) sin(θ 2 ) = (1.00/1.44)*sin(22 ) = θ 2 = sin 1 (0.260) =

10 Reflecting Light We now understand that light bends when it travels through a different medium. Let s combine this with our knowledge of reflections. When we look at a mirror, light from an object reflects off of a mirror.

11 Reflecting Light Light from an object bounces off of a mirror, obey the law of reflection, and our eyes see these reflections. However, these rays almost appear to be coming from behind the mirror. The object we see behind the mirror is called an image.

12 Reflecting Light

13 Reflecting Light However, things start to get a little strange once we bend the mirror. By bending the mirror, the image can appear in a different location or different size.

14 Reflecting Light We can see this in these examples:

15 What is a Lens? A lens is a piece of transparent material, such as glass, that refracts light. A lens forms an image by bending rays of light that pass through it. Where are lenses in this room?

16 Lenses Lenses can be found all over the place:

17 Lenses All of these objects bend light in specific ways. You already know the fundamental science principles that govern the bending of these objects (refracting and Snell s Law). Let s take a closer look.

18 Convex Glass Surface AIR (fast) normal GLASS (slow) fast to slow bends towards the normal C axis A convex surface is called converging because parallel rays converge towards one another

19 Convex Glass Surface normal GLASS AIR slow to fast bends away from the normal axis C The surface is converging for both air to glass rays and glass to air rays

20 Concave Glass Surface AIR GLASS axis C A concave surface is called diverging because parallel rays diverge away from one another

21 Concave Glass Surface GLASS AIR C axis Again, the surface is diverging for both air to glass rays and glass to air rays

22 Types of Lenses Converging Lens bi-convex Has two convex surfaces Diverging Lens bi-concave Has two concave surfaces

23 Types of Lenses A converging lens, or a convex lens, is thicker in the middle, and causes rays of light that are initially parallel to CONVERGE at a single point called the focal point. Focal point

24 Converging Lens F F Note that a lens has a focal point on both sides of the lens, as compared to a mirror that only has one focal point

25 Similarly to a spherical mirror, incoming parallel rays are deflected through the focal point Converging Lens F

26 Example What is an example of a converging lens? A magnifying glass

27 Applications of Converging Lenses Another application is inside of a camera. A camera uses a lens to focus an image on photographic film.

28 Types of Lenses A diverging lens, or a concave lens, is thinner in the middle, causing the rays of light to appear to originate from a single point. F F

29 Diverging Lens F F With a diverging lens, parallel rays are deflected such that when extended backwards, they appear to be coming from the focal point on the other side.

30 Examples What is an example of a diverging lens? A security mirror

31 Short and Far Sightedness We all have converging lenses inside of our eyes.

32 Near and Far Sightedness For those of you that are nearsighted, your eyeball is too long and images focus in front of the retina.

33 Near and Far Sightedness To correct the way the light lands on your eye, a concave lens acts to expand the focal length.

34 Near and Far Sightedness When someone is farsighted, the eyeball is too short, so the image gets focused behind the retina.

35 Near and Far Sightedness To fix this, convex lenses are used to shorten the focal length.