dagg3r
1. Calculate the wavelength of an x-ray which has a frequency of 10^16 Hz

2. Calculate the Energy associated with an x-ray with a frequency from Question 1.

* I don't know how to do this, but from looking at the electromagnetic spectrum, it tells you. But i want to know how to calculate it.

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A radioactive source (with a half life of 5 days) has an initial activity of 4000 counts/min.
3. Determine the activity after 10 days
4. If the initial quantitiy of radioactive material is 200gms, determine the amount left after 15 days have elapsed.
5. Convert 4000 counts per min into Bq's

* I know on half-life, the amount of is halved, but the activity is 4000 counts/min. so i divided 4000/60 = hrs, then divide by 24 to give days, then divide by 10 to give 10 days. but the half life is 5 days so not really sure what to do, someone can please show me how to do question 3,4,5

Last edited:

Staff Emeritus
Gold Member
dagg3r,

This looks suspiciously like your homework. We don't mind helping people with their homework (we have a whole forum devoted to Homework Help. The moderator is a hell of a guy, by the way. ). However, we typically like to see how you started and where you got stuck. I will ask you to answer some questions that will help you.

Originally posted by dagg3r
1. Calculate the wavelength of an x-ray which has a frequency of 10^16 Hz

*At what speed to x-rays travel?
*How are the wavelength, frequency and speed related?

2. Calculate the Energy associated with an x-ray with a frequency from Question 1.

*How are the energy and frequency of a quantum (aka "a photon") of EM radiation related?

* I don't know how to do this, but from looking at the electromagnetic spectrum, it tells you. But i want to know how to calculate it.

No, you do not need to look at the spectrum. You have everything you need here, except two universal constants which you must look up.

A radioactive source (with a half life of 5 days) has an initial activity of 4000 counts/min.
3. Determine the activity after 10 days
4. If the initial quantitiy of radioactive material is 200gms, determine the amount left after 15 days have elapsed.
5. Convert 4000 counts per min into Bq's

* I know on half-life, the amount of is halved, but the activity is 4000 counts/min. so i divided 4000/60 = hrs, then divide by 24 to give days, then divide by 10 to give 10 days.

Why did you divide by 60 hours?

but the half life is 4 days so not really sure what to do, someone can please show me how to do question 3,4,5

10 days=2 half lives
15 days=3 half lives

That's all I'm going to say for now. Give this a try and if you are still stuck, post what you've done and we'll take it from there.

dagg3r
Quote:
*At what speed to x-rays travel?
x-rays travel at the speed of light, All electromagnetic radiation travels at the speed of light in a vacumn i think.

*How are the wavelength, frequency and speed related?
well wavelength is the variation in colour or shade is caused by light. I think it is the combinatoin of an electric field and a magnetic field.

i think the energy and frequency of EM radiation is the same??? but anyways how do i calclate the wavelength of an x-ray which as a frequency of 10^16 hz. since i know the speed of light is 3 * 10^8.
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ok since you have told me, since 5 days is 1 halflife, 10 days is 2 halflives which i alread knew. i have to find the activity after 10 days since the initial activity is 4000 counts per minute. I'm guessint i just do 4000/2 = 2000 counts per min?

for q5 where it says initial quantitiy of material is 200gms, determine the amount left after 15 days have elasped.
i got 200/8 = 25gms, not sure if that is right...

how do i convert 4000 counts per min into bq's???

Staff Emeritus
Gold Member
I am with Tom, this sounds like homework to me also.

consider this

speed has units of m/s

wavelength is measured in meters (m)

Frequency has units of Hertz or (1/s)

Can you see a relationship between these quanities by considering their units? Remember that any algebraic relationship between physical quanties must be reflected in the dimensions (units) of the physical quanities.

Staff Emeritus
Gold Member
Originally posted by dagg3r
Quote:
*At what speed to x-rays travel?
x-rays travel at the speed of light, All electromagnetic radiation travels at the speed of light in a vacumn i think.

Right.

*How are the wavelength, frequency and speed related?
well wavelength is the variation in colour or shade is caused by light. I think it is the combinatoin of an electric field and a magnetic field.

No, it is much more basic than that. Look in your book in the chapter on electromagnetic waves, or even mechanical waves. You should have learned this some time ago.

i think the energy and frequency of EM radiation is the same??? but anyways how do i calclate the wavelength of an x-ray which as a frequency of 10^16 hz. since i know the speed of light is 3 * 10^8.
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You'll need the formula I referred to above.

Use algebra before plugging in numbers. ;)
velocity = wavelength*frequency

Energy of photon=nhfrequency
n=integer

N = Noe-kt
where N = no. particles final, No = no. particles initial, k = decay constant, t = time.