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Cermin cekung berjari-jari 60 cm,di depannya di letakkan benda berjarak 100 cm,jarak bayangan yang terbentuk adalah … cm

cermin cekung berjari-jari 60 cm,di depannya di letakkan benda berjarak 100 cm,jarak bayangan yang terbentuk adalah … cm
a.3,33
b.33,34
c.42,86
d.60

Jawaban yang benar adalah c. 42,86 cm.

Pembahasan:
Cermin cekung bersifat mengumpulkan sinar (konvergen). Fokusnya positif.

Persamaan pada cermin cekung:
1/f = 1/So + 1/Si
Dimana:
f = fokus (cm)
So = jarak benda (cm)
Si = jarak bayangan (cm)

Dik:
So = 100 cm
R = 60 cm

Dit: Si
Jawab:
Cari fokus (f) terlebih dahulu
R = 2f
60 = 2f
f = 60/2 = 30 cm

Kemudian cari jarak bayangan (Si) dengan menggunakan persamaan:
1/f = 1/So + 1/Si
1/30 = 1/100 + 1/Si
1/Si = 1/30 – 1/100
1/Si = 10/300 – 3/300
1/Si = 7/300
Si = 300/7 = 42,86 cm

Jadi, jarak bayangan yang terbentuk adalah c. 42,86 cm.

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