Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 5

2 Physics 1230: Light and Color Lecture 5: Shadows, eclipses. Reading: Chapter 2, pgs HW due on Thursday IR camera demo/video

3 Chapter 2 – Geometrical Optics 1.Shadows 2.Reflection 3.Refraction 4.Dispersion We are here Geometrical optics is the theory of RAYS (straight lines) and how they reflect and refract (bend). Main Topics

4 1. Shadows a.Point source or diffuse source? b.Umbra and penumbra c.Solar and lunar eclipses d.Pinhole cameras

Point source or diffuse source? Point-like source of light is small in size. Diffuse source of light has a large area. Size of a source is measured by angle to decide. 5 Sun looks smaller than a television

A point light source emits rays in all directions radially outwards The rays from two point light sources look like this The rays only tell us which direction the light goes in. We know that the light gets dimmer as you move further away from the light source. (Think of the sun. It would be blinding if we were closer to the sun)

Each point on the object emits or reflects rays in all directions (unless the object is a mirror) Incident rays from a frosted light bulb MANY reflected rays come from each point on Alex. This is diffuse reflection

Rays that ARE blocked by the book Shadows appear when rays are blocked Point light source Book Wall Shadow Rays that are NOT blocked by the book If we move the wall back from the book, the shadow gets… A)Smaller B) Bigger C) no change In your notebook, draw the rays that are and are not blocked by the book. Where’s the shadow?

Height of Gamow Tower Length of shadow Height of tower Length of shadow Height of you Height of you length of your shadow = Height of tower length of tower shadow Real height: 41.5 m

Notice that a shadow on the wall is the same situation, flipped Wall Height of you Height of your shadow flashlight

Shadows appear when rays are blocked 2 point light sources Book Wall A B umbra bright penumbra penumbrabright unblocked blocked The two parts of the penumbra each get light from only one of the two bulbs. umbra The umbra gets no light from either of the two bulbs. The bright region gets light from both of the bulbs. What else did you notice about the shadows outside?

umbrapenumbra We can extend the definition of the umbra and penumbra to exist in space even without a wall or screen! Book A B umbra bright penumbra penumbra Wallbright The light from B doesn't reach this penumbra The light from A doesn't reach this penumbra

Concept question Shadows tell us: A) What direction the light is shining from B) That something is blocking the light C) That light travels in straight lines D) A, B, & C E) A & C

We can think about large light sources as being composed of many small light sources

The more rays that reach a point the brighter the point Regions outside the penumbra and umbra are brighter – Becausehese regions get light rays from both point light sources The more lights you turn on the brighter the reflected light from objects in the room Light source 2 Light source 1 Reflected rays from light 1 Reflected rays from light 2 Your eye sees a brighter nose than with either light source alone

An extended light source such as the sun (or a large light bulb) also produces an umbra and penumbra Sun The umbra gets no light from any portion of the sun The penumbra gets light from part of the sun – If you look back from the penumbra you can see part of the sun A B Umbra Penumbra Rays from this part of the sun DO reach the upper penumbra Rays from this part of the sun DON'T reach the upper penumbra because they are blocked by Earth

Schedule: Sun, earth and moon aligned Moon entirely within umbra = total eclipse Viewed anywhere on night side of earth Why does the moon look red during an eclipse?

Lunar eclipse (partial and total) As we look at the moon, the shadow of the Earth is covering the moon. The Sun, Earth, and Moon must be lined up just right, in order for a lunar eclipse to take place. This happens very rarely. In most years there are only two lunar eclipses that can be seen only from certain places on Earth. In a partial lunar eclipse, the moon passes through the penumbra or part of the umbra. In a total lunar eclipse, the moon is completely within the umbra.

Based on what we know about eclipses, how do we prove this?

Umbra and penumbra In the umbra, all light from the source is blocked In the penumbra, some light is blocked. 20 Sun Moon Umbra Penumbra What you see at the different positions:

Solar eclipse Geometry: NOTE: The umbra is usually about 200km wide moon

Solar eclipse Solar eclipse - shadow of the Moon passes over the surface of the Earth. From the Earth, we see the moon blocking the light of the Sun. The Sun, Earth, and Moon must be lined up just right This happens 2-5 times a year. Since the Moon's shadow is so small, compared to the size of the Earth, a solar eclipse can be seen from only small portions of the Earth.

Map of the total solar eclipse, Aug 1,

A pinhole camera works by blocking rays Pinhole Camera Light bulb Image of light bulb blocked rays What will the image look like? Using shadows… This image is upside down and left-right reversed. In your notebook, draw the rays coming from the lightbulb. What do you think will happen? Pinhole film cans

Another prediction What do you think you will see? Pinhole Camera Image of two bulbs But let’s go outside and see stuff for real

The object photographed with a pinhole camera does not have to be self-luminous! Pinhole Camera Alex Image of Alex blocked rays Once again this image is upside down and left- right reversed. Note the correspondence between the distances object-camera-screen and image vs. object sizes One of many rays of light shining on Alex Reflected rays off the real Alex go through the hole and make the image

Finding an image by using rays is called ray tracing Trace rays from the object through the pinhole in the camera to find the image rather than trusting your intuition! Is the image of Alex smaller or larger than the real Alex? a)Smaller b)Larger c)Same size Is the image of Alex smaller or larger than the real Alex? a)Smaller b)Larger c)Same size

Construct and use a camera on your own (see textbook for details, pages 35 & 36)