1) A laser dazzles the audience at a rock concert by emitting red light with a w

Question

1) A laser dazzles the audience at a rock concert by emitting red light with a wavelength of 650 nm. Calculate the frequency of the light.

2) A 100-watt lightbulb radiates energy at a rate of 100 J/s (The watt, a unit of power, or energy over time, is defined as 1 J/s). If all of the light emitted has a wavelength of 510nm, how many photons are emitted per second? (Assume three significant figures in this calculation.)

3) What is the velocity of an electron that has a de Broglie wavelength approximately the length of a chemical bond? Assume this length to be 3.0×1010 m .

Explanation / Answer

1)

we know that

frequency (f) = speed of light (c) / wavelength

so

f = 3 x 10^8 / 650 x 10-9

f = 4.615 x 10^14

so

the frequency of the light is 4.615 x 10^14

2)

now

number of photons = Epulse / Ephoton

here

Ephoton = hc / lamda =6.626 x 10-34 x 3 x 10^8 / 510 x 10-9 = 3.89765 x 10^-19

Epulse = power x time = 100 x 1 = 100

so

number of photons = 100 / 3.89765 x 10-19

number of photons = 2.5656 x 10^20

3)

we know that

according to de broglie

wavelength (lamda) = h / m x v

here

mass of electron (m) = 9.1 x 10-31

h = plancks constant (h) = 6.626 x 10-34

so

3 x 10-10 = 6.626 x 10-34 / ( 9.1 x 10-31 x v)

v = 2.427 x 10^6

so

the velocity of the electron is 2.427 x 10^6 m/s

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