Test 2 - Light Waves

Why does the Sun look red at dawn and sunset? Explain in words and diagram.

Red scatters the least. So at sunset and sunrise it is the color that survives as the sun rays travel through a much thicker air layer to reach us.

Who and when first demonstrated light interference? What is the significance of the double slit experiment in the history of light?

Thomas Young - 1801 it determined that live is a wave phenomenon

What is the age of the universe? How do we find this age?

???

Why does the sky look blue? Explain well in words and diagram

Blue scatters the most. So the scattered light we receive from the sky is mostly blue and the sky looks blue.

Why light was considered to be a longitudinal wave?

Light was assumed to be longitudinal waves transmitted by ether. Ether was thought to be a gas and, since gases can transmit longitudinal waves only, light had to be such.

Why do we use diffraction gratings rather than color dispersion through prisms to analyze the spectrum of a light ray?

Passing light through a diffraction grating we obtain the spectrum of a light source. The spacing between lines depends on the color used.

How are the recession speeds of galaxies related to their distances? Draw the corresponding diagram.

The greater distance between any two galaxies, the faster they move apart. diagram - ???????

Draw a diagram showing light interference after light passes through a diffraction grating. Explain in words why we see, say, green at some angle θ

The longer the wavelength (λ) the greater is the angle θ for which the interference of the light rays produces a bright line of that color

Visible light and radio waves are both electromagnetic waves. Explain why visible light does not propagate around buildings while radio waves do

The longer the wavelength the greater the diffraction (spreading) of waves.???????

How does the degree of diffraction depend upon the wavelength of light and the width of the aperture? Draw two diagrams showing waves passing through apertures that have different widths.

The narrower the aperture, and longer the wavelength the greater the spreading.

Draw a diagram that explains the results of the double slit experiment according to the wave theory of light. Explain in words and in the diagram why there is a bright line at the center C on the screen.

The two light waves emerging from the slits overlap producing a succession of bright and dark lines, and travel equidistance, meet at point C , producing a strong wave, therefore, a bright line.

What color scatters the most and what the least by the molecules of the air? Explain why

Violet and blue colors scatter the most by the air molecules because their wavelengths (about 400nm) are closer to the sizes, about 1nm, of air molecules. Red colors scatter the least.

How do we obtain the wavelength of the color used in the double slit experiment? Draw the corresponding diagram.

We can obtain the wavelength of the color in the double-slit experiment by measuring the distance between the bright lines. The distance is greatest for red and least for violet.

What is the "Doppler effect" in light? Draw a diagram with three similar light sources, one approaching, one at rest, and one receding from an observer. Show the changes in the light wavelengths and the terms used to describe these changes.

doppler effect - is the apparent change in wavelength (frequency) of a wave due to the relative motion of wave source and observer. approaching - blue shift at rest - no shift receding - red shift

What is light diffraction? Why the diffraction of visible light passes undetected?

light diffraction - the bending of light past the edges of objects Wave diffraction is observed and measured best when the wavelength is long, and the wavelength of visible light is extremely short.

How do we know that the Universe is expanding?

light emitted by the stars of galaxies show red shifts. Also, the greater distance between any two galaxies, the faster they move apart. Thus we can tell that Galaxies recede from each other and the universe is expanding.

Why does the Sun look dimmer at dawn and sunset? Explain in words and diagram

looks dimmer - at sunset we see the sun through a much thicker layer of air which scatters away most of the sunlight, therefore it looks dimmer

Describe the polarization experiment, without showing light waves just light rays. What does the polarization experiment reveal about the nature of light?

polarization experiment - A light beam passing through two Polaroid filters can be stopped completely by turning one of the polaroids. The fact that light exhibits polarization indicates that light is a transverse wave

How do we determine the radial velocity (of approach or recession) of a star? Draw the corresponding diagram also showing the star spectra.

radial velocity - when analyzing the spectrum of a star, we can see if there is no shift, blue shift, or red shift. From these 'shifts', we can tell the radial velocity of the star relative to the earth. blue is approaching, red is receding

Draw a diagram that shows the effects produced on light waves when an unpolarized light ray passes through two aligned Polaroid filters.

see doc 3 page 3-15

Draw a diagram showing the Setup and Results in the double-slit experiment. Do not show any waves. What changes and how when we use various colors in the double slit experiment?

see doc 3 page 3-5 we see a bright line at the center between the slits surrounded by a succession of dark and bright lines. In addition, the spacing between the lines depends on the color we use. The spacing is greater for red and decreases gradually for orange, yellow, green, blue, and violet.

What are transverse waves and what are longitudinal waves? Draw appropriate diagrams.

transverse waves - the oscillations happen in a direction perpendicular to the direction of propagation of the wave longitudinal waves - Wave oscillations move back and forth along the direction of wave propagation.

Write down the range of wavelength of visible light. In particular, the wavelengths of violet, green, and red.

wavelengths of visible light are between 700nm and 400 nm violet - 400nm green - 550 nm red - 700nm

What is the degree of light scattering by the air molecules?

we can see clearly through air, therefore, we realize that light scatters very little by the air molecules.