Pinhole camera question with only two known variables?

  • Thread starter Thread starter ldesai149
  • Start date Start date
  • Tags Tags
    Camera Variables
Click For Summary

Homework Help Overview

The discussion revolves around a true or false question regarding a pinhole camera's image size and the object's actual size based on given distances. The problem involves the relationship between image height, object height, and distances using the magnification formula.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the application of the magnification formula and question the validity of using a specific value for the object's height without deriving it from the given information. There is an exploration of whether the problem can be solved with the provided variables.

Discussion Status

Participants are actively engaging with the problem, questioning assumptions about the object's height and discussing the implications of using specific values in calculations. There is a recognition of the need to derive the object's height from the given data rather than assuming it.

Contextual Notes

There is an emphasis on not using the height of the object as a known variable since it is not provided in the problem statement. The discussion highlights the importance of understanding the relationships between the variables involved in the pinhole camera setup.

ldesai149
Messages
8
Reaction score
0

Homework Statement



True or False: If a pinhole camera casts on the screen an image of 40.0cm in size of an object situated at a distance of 40.0m from the camera, then the size of the object is 10.0m.

Homework Equations



M = hi/ho = di/do

The Attempt at a Solution



hi = 40.0cm
do = 40.0m
ho = 10.0m (?)

M = hi/ho = 0.04m
di = dohi/ho = 0.16m
M = di/do = 0.004m

I'm guessing it is FALSE because the two values for M are different but I really have no idea how to go about a question like this. Any help would be greatly appreciated! :D
 
Physics news on Phys.org
Hello ldesai149,

Welcome to Physics Forums!
ldesai149 said:

Homework Statement



True or False: If a pinhole camera casts on the screen an image of 40.0cm in size of an object situated at a distance of 40.0m from the camera, then the size of the object is 10.0m.

Homework Equations



M = hi/ho = di/do

The Attempt at a Solution



hi = 40.0cm
do = 40.0m
ho = 10.0m (?)

M = hi/ho = 0.04m
di = dohi/ho = 0.16m
There's a mistake above involving what's in red. But read below before reaching a final conclusion in your answer. You used ho = 10 m as part of your calculations, but you shouldn't use a specific number for ho, because you don't know for sure what the object height actually is yet.
M = di/do = 0.004m

I'm guessing it is FALSE because the two values for M are different but I really have no idea how to go about a question like this. Any help would be greatly appreciated! :D
This is a true or false question. So at the time the statement is given, you don't really know what the object height ho is. Maybe it's 10 m if the statement is true. But maybe it's 12 m or 100 m, or some other height, if the statement is false. It's best not to use a specific value for ho unless you can derive that value first.

Since you don't know what ho is, is there a way to calculate it knowing only that hi = 0.40 m, and do = 40 m? If, based only on that information, you can calculate ho to be 10 m (without using "10 m" as part of your calculations), then the statement is true. If you calculate some other number or if there is simply not enough information to derive the 10 m figure, then the statement is generally false. :wink:
 
Last edited:
ldesai149 said:
hi/ho = di/do
Put another way, how many variables are in that equation? How many are given in the problem statement? Can this equation be solved with that information? (Hint: do not consider ho=10.0m as given; as collinsmark said, we do not know this.)
 
Thank you! :D It was a question on an exam and I couldn't stop thinking about it..
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 3 ·
Replies
3
Views
5K
  • · Replies 14 ·
Replies
14
Views
3K
Replies
3
Views
5K
Replies
4
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
Replies
3
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K