Convex lens questionCant find the equation i need. Please help

[sidge2222]

A convex lens of focal point 0.8m focuses a real image of the moon onto a screen. If the moon subtends an angle of 0.5 degrees to an observer on earth, calculate the size of the moons image on the screen?

Thats the question guys, I am really struggling to find the appropriate equation as we havnt been given any by out lecturer as he's been off ill. It needs be in after half term and we don't have any equations, i would greatly appreciate any help.

Im new to this forum so just wanted to say hi to everyone as well :)

Si

 

[sidge2222]

Because i have the focal point of 0.8, I am assuming that i should use the normal lens equation of 1/f = 1/u + 1/v which would be rearranged to 1/v = 1/f - 1/u because v = lens to image, so because I am after the size of the moons image on the screen this is what I am thinking is the correct equation. Am i right in thinking this guys??

So if I am after v, i need to obtain u in order to complete the equation - so am i right in thinking i need a sort of equation in order to convert the 0.5 degrees into a relevant figure??

Looking forward to your replies :)

Si

 

[sidge2222]

bump - sorry

 

[ehild]

If you draw a picture you can find out how is the angle of view related to the ratio of the object height to the object distance and that of the image height to the image distance. As for the object distance, the Moon is so far away that the object distance can be considered infinity. Where will be the image than?

ehild

 

[sidge2222]

so if u is infinitie that means its 1 right?

then my complete equation will be:

1/v = 1/f - 1/u

1/v = 1/0.8 - 1/1

1/v = 1/0.25

v = 4m

This seem right guys?? I am unsure as I am missing the moon subtending an angle of 0.5 degrees, which too be honest I am quite unsure about

thanks for the reply so far and for everyone that's had a read :)

Si

 

[ehild]

1 is not infinite. At the limit u→∞ 1/u→0.


ehild