# What is the frequency of the photon?

1. Dec 18, 2005

### helpm3pl3ase

a photon of red light have a wavelength of 750 x 10^-9 meters. What is the frequency??

3x10^8 divided by 750x10^-9

=

4 x 10^-4

Would this be the correct way to do the problem??

Also how would I go about finding the energy of a photon when I am just given the frequency?

2. Dec 18, 2005

### Staff: Mentor

Right method but wrong answer. Check your arithmetic, especially how you handle the exponents.

The energy of a photon is given by $E = h f$, where h is Plank's constant. (Look it up.)

3. Dec 18, 2005

### helpm3pl3ase

When I punch it into the calculator thats the answer I recieve?

4. Dec 18, 2005

### Staff: Mentor

Do it again, more carefully. If your calculator allows parentheses, use them: (3x10^8)/(750x10^-9).

Or you can handle the exponents separately and just use the calculator for the other numbers. What's (10^8)/(10^-9)?

5. Dec 18, 2005

### helpm3pl3ase

Finally figure it out.. 225.. Thank you for your help.

I have another question about mirrors. The question states:

When parallel light hits a concave mirror made from a piece of a sphere, all of the light rays converage at a point 3.8 cm from the mirror. What is the radius of the sphere??

Wouldnt it be 7.6?? 3.8 x 2 = 7.6??

6. Dec 18, 2005

### Staff: Mentor

That's right: The focal length of a spherical mirror is half the radius.

7. Dec 18, 2005

### helpm3pl3ase

I love the faster responses :rofl:

I have another question... I know I am annoying, but I dont understand this stuff...

How long does it take for light to go 300 meters in water? The index of refraction of water is 1.33.

I understand I have to use the formula N=C/V. Would I use 3 X 10^8 divided by 1.33??

8. Dec 18, 2005

### Staff: Mentor

That will give you the speed of light in water. Then use the usual "Distance = speed X time" to find the time.

9. Dec 18, 2005

### helpm3pl3ase

3x10^8 divided by 1.33 = 225,563,909.8

Distance = speed x time

300 meters = 225,563,909.8 x time

300 meters divided by 225,563,909.8 = 1.33 X 10^-6

That look right??

Another Mirror Question:

The average distance from the surface of the Earth to the surface of the Moon is 3.80 X 10^5 kilometers. How many seconds does it take for a laser beam starting on the earth and reflecting from a mirror on the moon to make the round trip??

I dont even know where to start on that problem

10. Dec 18, 2005

### Staff: Mentor

Looks good to me. (Don't forget your units.)

Just a note: Whenever possible, what until the very last step before reaching for the calculator--that will save you time. For example, here's how I would solve that problem.
(1) V = c/n = 3x10^8/1.33 (leave it just like that for now)
(2) T = D/v = (300)/(3x10^8/1.33) = [(300)/(3x10^8)]x 1.33 = 1.33 10^-6.
(I don't even need a calculator!)

Just use "Distance = speed x time" again. (What's the round trip distance?)

11. Dec 18, 2005

### helpm3pl3ase

Thanks for the tip, it helped me a bunch and saved some time.

For the moon problem:

3.8x10^5/3x10^8 = 1.27 x 10^13

Look and seem right???

Mirror question 2:

A near sighted snake, 17 feet long, lies perpendicular to and facing, a flat mirror. The snake cant see anything farther away than 25 feet from its eyes. What is the largest distance from its face to the mirror at which it can see the image of its tail??

The answer is 8 correct?? No tricks involved??

12. Dec 18, 2005

### Staff: Mentor

Not quite right. Use the total distance: Earth to Moon and back.

Not quite correct. Hint: If you stand 5 feet from a mirror, how far away does your image appear?

13. Dec 18, 2005

### helpm3pl3ase

(1.27 x 10^13)^2 = 1.6129 X 10^26?? Hows that look??

For the mirror question:

If you stand 5 feet infront of a mirror your image will appear to be 10 ft away.

Answer: The snake can stand 12.5 feet away from the mirror?? Is that correct???

14. Dec 18, 2005

### Staff: Mentor

Pretty crazy! Why in the world did you square that answer? Hint: If the distance to the moon is D, what's the distance to the moon and back? (Or: If it takes x seconds to get to the moon, how many seconds does it take to get to the moon and back?)

Right!

Nope. If he "stood" 12.5 feet from the mirror, the image of his head would appear 25 feet away. Which means he'll just barely see his head, never mind his tail! (If he were 12.5 feet away from the mirror, how far away would the image of his tail be?)

15. Dec 18, 2005

### helpm3pl3ase

I dont know why I squared it.. Iam suppose to double it, I see this now

1.27x10^13 X 2 = 2.54x10^13

Ok so the snake would have to be 4 feet away from the mirror??

New problem: A proton is moving with a velocity of 2.2 X 10^4 meters per second. What is the deBroglie wavelength of this proton? The mass of the proton is 1.67x10^-27 kg and its charge is +1.6x10^-19 Coulombs.

I would use the formula: Wavelength = Planks constant/(Mass)(Velocity)

Is that correct??

16. Dec 19, 2005

### Staff: Mentor

OK

Right!

That is correct.

17. Dec 19, 2005

### helpm3pl3ase

Thanks for you help. I really appreciate it!!

18. Dec 19, 2005

### Staff: Mentor

Your calculator should have a way of entering numbers in scientific notation without entering the "x 10" explicitly. For example, on one of my calculators, this is done with the "EE" button, so that to enter the speed of light I would enter:

3 EE (-) 8

This calculator has separate keys for entering a negative number "(-)" versus the subtraction operation "-".

On another calculator, the sequence is

3 EXP CHS 8

where "EXP" is the same as "EE" on the first calculator, and "CHS" changes the sign of the following number (makes it negative).

Whatever method your calculator uses, it's a good idea to learn it. People use scientific notation a lot in physics!