Light
Electromagnetic waves, color, reflection, and refraction
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Narrated Lesson
showmethephysics.com
Resonance Review
UC Irvine
School blocks YouTube?
Bubbles
A. Speed
1.
3 x 108 m/s = c (in vacuum)
(fastest speed in universe) In air v ≈ 3 x 108 m/s
v = fλ
c = fλ
3 x 108 m/s = fλ
Divide both sides by λ
f = c/λ
Frequency, Wavelength Relationship?
Inverse
Ex) What is the wavelength of light with a frequency of 7.69 x 1014 Hz in a vacuum?
c = fλ
3 x 108 m/s = 7.69 x 1014 Hz λ
λ = 4 x 10-7 m
2. Speed of light depends on the medium
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Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. What is the speed of light in a vacuum?
2. What is the wavelength of light with a frequency of 6.0 × 10¹⁴ Hz in a vacuum?
3. For light in a vacuum, how are frequency and wavelength related?
4. The speed of light depends on
5. Sunlight reaches Earth through the vacuum of space, but sound from the Sun cannot. Why?
How Long Does Sunlight Take to Reach Us?
Light travels through the vacuum of space at 3.00 × 108 m/s. The Sun is about 1.5 × 1011 m from Earth.
About how long does light take to travel from the Sun to Earth?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. The Moon is 3.84 × 108 m from Earth. Substitute numerical values into a mathematical representation that can be used to determine the time for light reflected from the Moon to reach Earth. Then calculate the time.
C2. Red light has a wavelength of 7.0 × 10−7 m in a vacuum. What is its frequency?
C3. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C4. During a storm, a student sees a flash of lightning and hears the thunder 6.0 s later. The speed of sound in air is 340 m/s. Determine the distance to the lightning strike. (The light arrives almost instantly.)
C5. During a thunderstorm, you see lightning before you hear thunder. Why?
C6. Astronomers often say that looking at the Sun or a distant star is “looking back in time.” Use evidence from the scenario to explain what they mean.