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わかっているようでわからない凸レンズ | 夢を叶える塾
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わかっているようでわからない凸レンズ

ホームページのリニューアルに伴い一部のページは新しいサイトにリダイレクトされます。

 

凸レンズ

中学生の理科で光を学ぶ際に出てくる凸レンズ。
よく光の道すじを書かせる問題が出題されます。

焦点

凸レンズの焦点が作図のうえで重要になってきますね。
では焦点って何だったか?
「そんなの簡単!光の集まるところだよ!」
本当にそうでしょうか?

そう書くだけあって、これだけでは不正解です。
辞書などには次のように記載されています。

反射鏡・レンズに平行に入射し、反射・屈折した光線が集まる点。

「平行に入射」というのがポイントです。
これは入射角(垂直から何度ずれているか)が0度という事です。
平衡に入射している光は次の様に一点で交わり、それが焦点です。
焦点1

実像

では平行に入射しない光はどのように進む(屈折)するのでしょうか?
これには二本の光線の作図が必要になります。
一本は平行に入射する光です。
これは焦点を通るように屈折します。
もう一本はレンズの中心を通る光です。
これは直進します。
すると、凸レンズの焦点よりも遠い光源の光は、凸レンズ越しの焦点より遠い点で交わります。
そして、同じ光源の光は、すべてこの点で交わります。
焦点2

このようにしてある光源の光がすべて集約されるところに実像ができます。
発する光が再度集約するからくっきりと見えるわけです。

凸レンズを一部隠すとどうなるか?

先の図で凸レンズの下半分を隠すとどうなるでしょうか?
ここまでしっかり凸レンズについて理解できていれば簡単ですね。
次の図の様になります。
焦点3

このようにして下半分を通る光がさえぎられます。
発する光の一部が失われるので、実像は暗くなります。

終りに

ここまでの光と凸レンズの性質を押さえておけば、
「焦点距離の二倍の位置で実像は同じ大きさになる」
ですとか、
「光源を凸レンズに近づけると実像は大きくなる」
とか、作図するだけで求める事が出来るようになります。
暗記する必要が無く、間違って覚えることも無く、とても楽に正解する事ができるようになります。
しかし一点で再び光が収束するというのは不思議かもしれません。
実像のできる点から、平行に入射する光を作図すると、面白い事がわかるかもしれません。

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