Figuring out what image distance (lens and mirror) formula to use?

  • Context: Undergrad 
  • Thread starter Thread starter Clomo4star
  • Start date Start date
  • Tags Tags
    Formula Image Mirror
Click For Summary

Discussion Overview

The discussion revolves around the formulas related to image distance in lens and mirror equations, specifically derived from the lensmaker's formula. Participants are exploring how to distinguish between different forms of the equations and the derivation process for one of them.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents two derived equations from the lens formula, questioning how to distinguish between them and seeking clarification on the derivation of the second equation.
  • Another participant suggests focusing on understanding the physics rather than memorizing equations and claims the first equation presented is incorrect.
  • A different participant questions the correctness of the right-hand side of the first equation and proposes an alternative formulation, indicating it as a rearrangement of the original equation.
  • One participant expresses interest in the second equation specifically and acknowledges a mistake in their earlier message.
  • Another participant encourages further exploration of the derivation process for the second equation.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the first equation, with some asserting it is incorrect while others question its formulation. The discussion remains unresolved regarding which equation is appropriate in different contexts.

Contextual Notes

There are unresolved assumptions regarding the derivation steps and the conditions under which each equation applies. The discussion does not clarify the specific scenarios for using each formula.

Clomo4star
Messages
3
Reaction score
0
I was practicing homework and I came across these two derived equations from 1/f=1/d0+1/di
1st one 1/di=1/d0-1/f
2nd one di=d0(f)/d0-f
How do distinguish which one to use? and how did they get that derivation for the 2nd equation?
 
Science news on Phys.org
I wouldn't worry about memorizing the different forms of the equations to use for which situation, focus on understanding the physics.

Note - the second one is just what happens to the lensmakers formula if you solve it for di. The first one is incorrect.
 
Clomo4star said:
I was practicing homework and I came across these two derived equations from 1/f=1/d0+1/di

1st one 1/di=1/d0-1/f

Are you sure the right-hand side wasn't reversed, that is,

$$\frac{1}{d_i} = \frac{1}{f} - \frac{1}{d_o}$$

2nd one di=d0(f)/d0-f

This is just a re-arrangement of your first equation. It's good algebra practice. First, get the 1/di all by itself on the left as in my equation above. Can you see where to go from there? (hint: how do you add or subtract fractions?)
 
Oh yeah sorry oops I was just wondering about the 2nd one.
 
No worries - did you have a go at the derivation yet? How did you get on?
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 17 ·
Replies
17
Views
4K
  • · Replies 13 ·
Replies
13
Views
11K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 7 ·
Replies
7
Views
5K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K