Teacher’s Notes: Digital Image Sources Digital images from space can be found at the following NASA web site: Click the map for images of that area. You can include in your search results information such as Altitude and Focal Length of the camera used by selecting FCLT and ALT at the bottom of the page. Images can be used freely if attributed to NASA. For added impact Google Earth ( can be used to zoom from space to Tower Bridge, the Channel, etc. Depending on your location you may find better resolution images of your town on Microsoft’s Live Local site: Information on aerial photography:
Starter How many, a.mm in 20 cm? b.cm in 3.75 m? c.m in 2.5 km? d.cm in 6 km? e.mm in 8 km?
Similar Triangles & Photography
Enlargement Factor = x = 20 cm = x = × 8 cm 5252 Reminder x cm 8 cm 10 cm 25 cm Find the value of x.
Exercise In each case, the pair of triangles are similar. Equal angles are marked. Find:ithe relevant scale factor (enlargement or reduction) iithe missing length. 12 cm x cm 7 cm 8 cm b a 10 m 5 m x m 3 m Answers:aiii10.5 cm biii6 m
Lens Distance to object Focal Length Object Height Image Height How a Lens Works
Lens Distance to object 10 m Focal Length 35 mm Object Height x Image Height 14 mm Example
Enlargement Factor = x = 4000 mm x = 4 m 10 m 35 mm mm 35 mm = = x = × 14 mm
Satellite Photography Image: NASA
Lens Height above ground (Altitude) Focal length of camera Object size Image size Similar Triangles in Space
Lens 135 km 90 mm x 20 mm Example EF = 135 km 90 mm mm 90 mm = x = mm x = 30 km x = = × 20 mm
Invergordon Academy Playing Fields Aerial Photo Altitude = 400 m Focal length = 152 mm Is the 100 metre track marked correctly?
Tower Bridge London Aerial Photo Altitude = 2500 ft (762 m) Focal length = 152 mm Calculate the span of the bridge.
English Channel Photo taken onboard Space Shuttle Altitude = 126 miles (203 km) Focal length = 90 mm Calculate the shortest distance across the Channel. (marked) Image: NASA
Actual Distances Tower Bridge – 244 m English Channel – 20 miles or 32 km