Hey all, I posted a question earlier about focal length and now I\'mtrying to dr

Question

Hey all, I posted a question earlier about focal length and now I'mtrying to draw a ray diagram..

Here's the original question:

You wish to see an image of yourself in a mirror for applyingmakeup. You would like to see yourself standing upright andtwice as big when you are 30 cm from a mirror. What would thefocal length of such a mirror be?

For that, I got 60 cm, but shouldn't it be negative since it's aconvex mirror?

Then, it says: For the case described above, draw a ray diagram toscale to locate the image. Assume your face to be 15 cmlong. The vertical line is the mirror, and its straight sincewe are ignoring curves.

Any help with the drawing would be great! Thanks so much!

Explanation / Answer

Hello, A makeup mirror with an upright and magnified image would have tobe Concave, yet you would have to have the object inside of thefocus f. So if the object distance is do = 30, and themagnification is +2 (M = -di/do), we have di = -2do or -60. This means that there is a virtual image which appears on the otherside of the mirror ("in" the mirror). So 1/di + 1/do = 1/f, 1/-60 + 1/30 = 1/f f = 60 cm. The reason you don't get a negative is because themagnification is +2 but it is -di/do. Here's an image showing what it might look like: http://polaris.deas.harvard.edu/galileo/images/ansimages/5675/ConcaveMirrorInsideF_answer.jpg

Related Questions

Navigate

• Browse (All)
• Browse H
• Subjects

---